Method and device for decoding image by using partition unit including additional region

ABSTRACT

The invention relates to a method and a device for decoding an image by means of a partition unit comprising an additional region. The method of decoding an image by means of a partition unit comprising an additional region comprises the steps of: partitioning, by reference to a syntax element acquired from a received bit stream, an encoded image, which is included in the bitstream, in at least one partition unit; define an additional region for at least one partition unit; and decoding the encoded image based on the partition unit for which the additional region is defined. The invention improves the image coding efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/KR2018/007520, filed on Jul. 3, 2018, which claims priority fromKorean Patent Application No. 10-2017-0084517, filed on Jul. 3, 2107,Korean Patent Application No. 10-2017-0090638, filed on Jul. 17, 2017,Korean Patent Application No. 10-2018-0055980, filed on May 16, 2018,Korean Patent Application No. 10-2018-0076918, filed on Jul. 3, 2018,and Korean Patent Application No. 10-2018-0076932, filed on Jul. 3,2018.

TECHNICAL FIELD

The present invention relates to an image decoding method and apparatususing a division unit including an additional area. More specifically,the present invention relates to a technique for improving codingefficiency by setting additional areas up, down, left, and right for thedivision unit such as a tile in an image, and encoding by referring toimage data in the additional area.

BACKGROUND ART

Recently, demands for multimedia data such as moving pictures is rapidlyincreasing on the Internet. However, the pace at which a bandwidth of achannel develops is increasing rapidly, and there is a need for a methodof efficiently compressing the amount of multimedia data. ISO/ISE MPEG(Moving Picture Experts Group) and ITU-T VCEG (Video Coding ExpertsGroup) are researching video compression standards through constantjoint research.

On the other hand, when performing independent encoding on an image,since encoding is generally performed independently for each divisionunit including a tile, there is a problem which is not possible to referto image data of another division unit adjacent in time and space.

Accordingly, there is a need for a method of referring to adjacent imagedata while maintaining parallel processing according to independentencoding as in the prior art.

In addition, an intra prediction according to an existing imageencoding/decoding method configures a reference pixel using pixelsnearest to a current block, but it may be not preferable to configurethe reference pixel as the nearest pixel depending on a type of animage.

Therefore, there is a need for a method of improving intra predictionefficiency by configuring the reference pixel unlike the conventionalmethod.

DISCLOSURE Technical Problem

An object of the present invention for solving the above problems is toprovide an image decoding method and apparatus using a division unitincluding an additional area.

Another object of the present invention for solving the above problemsis to provide an image encoding method and apparatus using a divisionunit including an additional area.

An object of the present invention for solving the above problems is toprovide an image decoding method supporting multiple reference pixellayers.

Another object of the present invention for solving the above problemsis to provide an image decoding apparatus supporting multiple referencepixel layers.

Technical Solution

One aspect of the present invention for achieving the above object is toprovide an image decoding method using a division unit including anadditional area.

Herein, a method for decoding an image using a division unit includingan additional area comprises dividing an encoded image included in areceived bitstream into at least one division unit by referring to asyntax obtained from the received bitstream, setting an additional areafor the at least one division unit, and decoding the encoded image basedon the division unit in which the additional area is set.

Herein, the decoding of the encoded image comprises, according toinformation indicating a possibility of referencing included in thebitstream, determining a reference block for a current block to bedecoded in the encoded image.

Herein, the reference block is a block belonging to a positionoverlapping with the additional area set in the division unit to whichthe reference block belongs.

Another aspect of the present invention for achieving the above objectprovides an image decoding method supporting multiple reference pixellayers.

an image decoding method supporting multiple reference pixel layers mayinclude checking whether a bitstream supports multiple reference pixellayers, determining a reference pixel layer to be used for a currentblock by referring to syntax information included in the bitstream ifthe plurality of reference pixel layers is supported, constructing areference pixel using a pixel belonging to the determined referencepixel layer, and performing intra prediction on the current block byusing the constructed reference pixel.

Herein, after the checking of whether the plurality of reference pixellayers are supported, the method may further include checking whetherthe bitstream supports an adaptive reference pixel filtering method.

Herein, after checking whether the plurality of reference pixel layersis supported, if the plurality of reference pixel layers is notsupported, the method may include configuring a reference pixel usingthe preset reference pixel layer.

Advantageous Effects

In the case of using the above-described image decoding method andapparatus using the division unit including the additional areaaccording to the present invention, since there is a lot of image datathat may be referred to, image compression efficiency may be improved.

In the case of using the above-described image decoding method andapparatus for supporting the plurality of reference pixel layersaccording to the present invention, since the plurality of referencepixels is used, an accuracy of intra prediction may increase.

In addition, according to the present invention, since adaptivereference pixel filtering is supported, optimal reference pixelfiltering may be performed according to characteristics of an image.

In addition, there is an advantage that the compression efficiencyaccording to image encoding/decoding may be increased.

DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of an image encoding and decoding systemaccording to an embodiment of the present invention.

FIG. 2 is a block diagram of an image encoding apparatus according to anembodiment of the present invention.

FIG. 3 is a block diagram of an image decoding apparatus according to anembodiment of the present invention.

FIGS. 4a to 4d are conceptual diagrams for illustrating a projectionformat according to an embodiment of the present invention.

FIGS. 5a to 5c are conceptual diagrams for illustrating a surfacearrangement according to an embodiment of the present invention.

FIGS. 6a and 6b are exemplary diagrams for illustrating a division unitaccording to an exemplary embodiment of the present invention.

FIG. 7 is an exemplary diagram in which one picture is partitioned intomultiple tiles.

FIGS. 8a to 8i are first exemplary diagrams for setting an additionalarea for each tile according to FIG. 7.

FIGS. 9a to 9i are second exemplary diagrams for setting an additionalarea for each tile according to FIG. 7.

FIG. 10 is an exemplary diagram of using an additional area generatedaccording to an embodiment of the present invention in anencoding/decoding process of another area.

FIGS. 11 to 12 are exemplary diagrams for illustrating anencoding/decoding method for a division unit according to an embodimentof the present invention.

FIGS. 13a to 13g are exemplary diagrams for illustrating an area inwhich a specific division unit may be referred to.

FIGS. 14a to 14e are exemplary diagrams for illustrating a possibilityof referencing for an additional area in a division unit according to anembodiment of the present invention.

FIG. 15 is an exemplary diagram of a block belonging to a division unitof a different image from a block belonging to a division unit of acurrent image.

FIG. 16 is a hardware block diagram of an image encoding/decodingapparatus according to an embodiment of the present invention.

FIG. 17 is an exemplary diagram for illustrating an intra predictionmode according to an embodiment of the present invention.

FIG. 18 is a first exemplary diagram of a reference pixel configurationused for intra prediction according to an embodiment of the presentinvention.

FIGS. 19a to 19c are second exemplary diagrams of reference pixelconfigurations according to an exemplary embodiment of the presentinvention.

FIG. 20 is a third exemplary diagram of a reference pixel configurationaccording to an embodiment of the present invention.

FIG. 21 is a fourth exemplary diagram of a reference pixel configurationaccording to an embodiment of the present invention.

FIGS. 22a to 22b are exemplary diagrams of a method of filling areference pixel at a preset position in an unavailable referencecandidate block.

FIGS. 23a to 23c are exemplary diagrams of a method of performinginterpolation based on a fractional pixel unit for reference pixelsconfigured according to an embodiment of the present invention.

FIGS. 24a and 24b are first exemplary diagrams for illustrating anadaptive reference pixel filtering method according to an embodiment ofthe present invention.

FIG. 25 is a second exemplary diagram for illustrating an adaptivereference pixel filtering method according to an embodiment of thepresent invention.

FIGS. 26a to 26b are exemplary diagrams of using one reference pixellayer in reference pixel filtering according to an embodiment of thepresent invention.

FIG. 27 is an exemplary diagram of using multiple reference pixel layersin reference pixel filtering according to an embodiment of the presentinvention.

FIG. 28 is a block diagram for illustrating a encoding/decoding methodfor an intra prediction mode according to an embodiment of the presentinvention.

FIG. 29 is a first exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

FIG. 30 is a second exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

FIG. 31 is a third exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

FIG. 32 is a flowchart for illustrating an image decoding methodsupporting multiple reference pixel layers according to an embodiment ofthe present invention.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, and theexemplary embodiments can be construed as including all modifications,equivalents, or substitutes in a technical concept and a technical scopeof the present invention. The similar reference numerals refer to thesimilar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, ‘A’, ‘B’, etc. canbe used to describe various components, but the components are not to beconstrued as being limited to the terms. The terms are only used todifferentiate one component from other components. For example, the‘first’ component may be named the ‘second’ component without departingfrom the scope of the present invention, and the ‘second’ component mayalso be similarly named the ‘first’ component. The term ‘and/or’includes a combination of multiple items or any one of multiple terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded.

Unless defined otherwise, all terms used herein, including technical orscientific terms, have the same meaning as commonly understood by one ofordinary skill in the art. Terms such as those defined in the commonlyused dictionaries should be interpreted as having meanings consistentwith those in the context of the related art. Unless expressly definedherein, they are not to be interpreted in an ideal or excessively formalsense.

In general, an image may be composed of a series of still images, andthese still images may be divided into groups of pictures (GOP), andeach still image may be called a picture or a frame. As a higherconcept, a unit such as a GOP, a sequence, or the like may exist, andeach picture may be divided into predetermined areas such as slices,tiles, blocks, and the like. In addition, one GOP may include a unitsuch as an I picture, a P picture, and a B picture. An I picture maymean a picture that is encoded/decoded by itself without using areference picture, and a P picture and a B picture use a referencepicture to perform a process such as motion estimation and motioncompensation. This may mean a picture to be encoded/decoded byperforming. In general, a P picture may use an I picture and a P pictureas a reference picture, and a B picture may use an I picture and a Ppicture as a reference picture.

Herein, a picture referred to for encoding/decoding is referred to as areference picture, and a referred block or pixel is referred to as areference block and a reference pixel. In addition, the reference datamay be not only pixel values in a spatial domain but also coefficientvalues in a frequency domain and various encoding/decoding informationgenerated and determined during an encoding/decoding process.

The minimum unit constituting an image may be a pixel, and the number ofbits used to represent one pixel is called a bit depth. In general, thebit depth may be 8 bits and may support different bit depths accordingto encoding settings. The bit depth may support at least one bit depthaccording to a color space. In addition, at least one color space may beconfigured according to a color format of an image. It may consist ofone or more pictures with a certain size or one or more pictures withother sizes, depending on the color format. For example, in case ofYCbCr 4:2:0, it may be composed of one luminance component (Y in thisexample) and two chrominance components (Cb/Cr in this example). In thiscase, a constituent ratio of the chrominance component and the luminancecomponent may have a ratio of 1:2 in the horizontal and verticaldirections. As another example, 4:4:4 may have the same configurationratio in the horizontal and vertical directions. When the picture iscomposed of one or more color spaces as described above, the picture maybe divided into each color space.

In the present invention, description will be made based on some colorspaces (Y in this example) of some color formats (YCbCr in thisexample). The same or similar application (setting dependent on aspecific color space) may be made for other color spaces (Cb, Cr in thisexample) according to the color format. However, it may also be possibleto make partial differences (settings independent of a particular colorspace) in each color space. That is, a setting dependent on each colorspace has a setting proportional to or dependent on the compositionratio of each component (for example, 4:2:0, 4:2:2, 4:4:4, etc.), and asetting independent of each color space may mean having a setting ofonly the corresponding color space irrespective of the composition ratioof each component or independently. In the present invention, dependingon the encoder/decoder, some components may have independent settings ordependent settings.

Setting information or syntax elements required in an image encodingprocess may be determined at the unit level of a video, sequence,picture, slice, tile, block, and the like, these may be included in abitstream and transmitted to a decoder in a unit such as a videoparameter set (VPS), a sequence parameter set (SPS), a picture parameterset (PPS), a slice header, a tile header, a block header, and the like.etc., and in the decoder, the setting information transmitted from anencoder by parsing in the same level unit may be restored and used in animage decoding process. In addition, related information may betransmitted in a bitstream, parsed, and used in the form of SupplementEnhancement Information (SEI), Metadata, or the like. Each parameter sethas a unique ID value, and a lower parameter set may have an ID value ofa higher parameter set to be referred to. For example, the lowerparameter set may refer to information of the higher parameter sethaving a matching ID value among one or more higher parameter sets. Aunit corresponding to a case in which any one of the examples of thevarious units mentioned above includes one or more other units may bereferred to as a higher unit, and the included unit may be referred toas a lower unit.

The setting information generated in the unit may include a content ofan independent setting for each corresponding unit, or may include acontent of a setting depending on a previous, subsequent, higher unit,or the like. Herein, the dependent setting may be understood asindicating setting information of the corresponding unit as flaginformation (for example, as a 1-bit flag, if the value of the flag is1, follow; if 0, do not follow) indicating that the dependent settingfollows the setting of a previous, subsequent, and higher unit. In thepresent invention, the setting information will be described based onthe example of the independent setting, but the example of adding orreplacing to a content about the relationship depending on the settinginformation of the previous, subsequent, or higher unit of a currentunit may be included.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram of an image encoding and decoding systemaccording to an embodiment of the present invention.

Referring to FIG. 1, the image encoding apparatus 105 and the decodingapparatus 100 may be a personal computer (PC), a notebook computer, apersonal digital assistant (PDA), a portable multimedia player (PMP), aplaystation portable (PSP), a wireless communication terminal, a smartphone, a user terminal such as a TV or the like, or a server terminalsuch as an application server, a service server or the like, and mayinclude various apparatuses including a communication device such as acommunication modem for performing communication with various devices orwired and wireless communication networks, memory 120, 125 for storingvarious programs and data for inter or intra prediction for encoding ordecoding an image, processors 110, 115 for operating and controlling,etc. In addition, an image coded into a bitstream by the image encodingapparatus 105 may be transmitted to the image decoding apparatus 100through wired and wireless networks such as the Internet, short-rangewireless communication network, wireless LAN network, WiBro network,mobile communication network, etc., or through various communicationinterfaces such as cables, Universal Serial Bus (USB), etc., inreal-time or non-real-time. Also, an image encoded in the bitstream bythe image encoding apparatus 105 may be transferred from the imageencoding apparatus 105 to the image decoding apparatus 100 through acomputer-readable recording medium.

FIG. 2 is a block diagram of an image encoding apparatus according to anembodiment of the present invention.

An image encoding apparatus 20 according to the present embodiment, asshown in FIG. 2, may include a prediction unit 200, a subtractor 205, atransformation unit 210, a quantization unit 215, an inversequantization unit 220, an inverse transformation unit 225, an adder 230,a filter unit 235, an encoded picture buffer 240 and an entropy encodingunit 245.

The prediction unit 200 may include an intra prediction unit performingintra prediction and an inter prediction unit performing interprediction. The intra prediction may generate a prediction block byperforming spatial prediction using a pixel of block adjacent to thecurrent block, and the inter prediction may generate a prediction blockby finding a area that most matches the current block from the referencepicture and performing motion compensation. Whether to useintra-prediction or inter prediction for the unit (coding unit orprediction unit) may be determined, and specific information (e.g.,intra-prediction mode, motion vector, reference picture, etc.) for eachprediction method may be determined. In this case, a processing unit inwhich the prediction is performed and a processing unit where theprediction method and concrete contents are decided may be determinedaccording to an encoding/decoding setting. For example, the predictionmethod, the prediction mode, etc. may be determined in a predictionunit, and the performance of the prediction may be performed in atransformation unit.

The intra prediction unit may have a directional prediction mode such asa horizontal and a vertical mode used according to a predictiondirection, and a non-directional prediction mode such as a DC and aplanar mode using a method of averaging or interpolating referencepixels. An intra prediction mode candidate group may be configuredthrough the directional and non-directional modes. One of a variety ofcandidates such as 35 prediction modes (33 directional+2non-directional) or 67 prediction modes (65 directional+2non-directional), 131 prediction modes (129 directional+2non-directional) may be used as a candidate group.

An intra prediction unit may include a reference pixel configurationunit, a reference pixel filter unit, a reference pixel interpolationunit, a prediction mode determination unit, a prediction blockgeneration unit, and a prediction mode encoding unit. The referencepixel configuration unit may configure a pixel belonging to aneighboring block around a current block and adjacent to the currentblock as a reference pixel for intra prediction. According to anencoding setting, one nearest reference pixel line may be configured asreference pixels, or another adjacent reference pixel line may beconfigured as the reference pixel, and multiple reference pixel linesmay be configured as the reference pixel. If a part of the referencepixel is not available, the reference pixel may be generated using theavailable reference pixel, and if all of the reference pixel is notavailable, the reference pixel may be generated using a preset value(e.g., median of the range of a pixel value represented by a bit depth,etc.)

The reference pixel filter unit of the intra prediction unit may performfiltering on the reference pixel in order to reduce deteriorationremaining through an encoding process. At this time, the filter used maybe a low-pass filter such as a 3-tap filter [¼, ½, ¼], a 5-tap filter [2/16, 3/16, 6/16, 3/16, 2/16], etc. Whether to apply filtering or afiltering type may be determined according to encoding information(e.g., a size, a shape, an prediction mode of a block, or the like).

The reference pixel interpolation unit of the intra prediction unit maygenerate a pixel in a decimal unit through a linear interpolationprocess of a reference pixel according to a prediction mode, and aninterpolation filter applied according to encoding information may bedetermined. In this case, the interpolation filter used may include a4-tap cubic filter, a 4-tap Gaussian filter, a 6-tap Wiener filter, an8-tap Kalman filter, and the like. Interpolation is generally performedseparately from a process of performing the low pass filter, but thefiltering process may be performed by combining the filters applied tothe two processes into one.

The prediction mode determination unit of the intra prediction unit mayselect an optimal prediction mode among a prediction mode candidategroup in consideration of an encoding cost, and the prediction blockgeneration unit may generate a prediction block using the selectedprediction mode. The prediction mode encoding unit may encode theoptimal prediction mode based on a prediction value. In this case,prediction information may be adaptively encoded according to the casewhere the prediction value is correct or the case where the predictionvalue is not correct.

In the intra prediction unit, the prediction value is referred to asMost Probable Mode (MPM), and some modes among all modes belonging to aprediction mode candidate group may be configured as an MPM candidategroup. The MPM candidate group may include a preset prediction mode(e.g., a DC, planar, vertical, horizontal, diagonal Mode, etc.) or aprediction mode of a spatially adjacent block (e.g., a left, top, topleft, top bottom block, etc.). In addition, a mode derived from a modepreviously included in the MPM candidate group (difference of +1, −1,etc. in the directional mode) may be configured as the MPM candidategroup.

There may be a priority of prediction mode for configuring an MPMcandidate group. The order of inclusion in the MPM candidate group maybe determined according to the priority, and when the number of MPMcandidates (determined according to the number of prediction modecandidates in the MPM candidate groups) is filled according to thepriority, the MPM candidate group configuration may be completed. Inthis case, a priority may be determined in order of a prediction mode ofa spatially adjacent block, a preset prediction mode, and a mode derivedfrom a prediction mode previously included in the MPM candidate group,but other modifications are also possible.

For example, they may be included in the candidate group in the order ofblocks, such as left—top—bottomleft—topright—topleft, among spatiallyadjacent blocks. In addition, the preset prediction mode may be includedin the candidate group in the order of a DC mode—a Planar mode—aVertical mode—a Horizontal mode. In addition, A total of six modes maybe configured as the candidate group by including modes obtained byadding +1, −1, and the like to a previously included mode. In addition,a total of seven modes may be configured as the candidate group byincluding modes with one priority such as the prediction mode of theleft block—the prediction mode of the top block—the DC mode—the Planarmode—the prediction mode of the bottom left block—the prediction mode ofthe top right block—the prediction mode of the top left block—(theprediction mode of the left block+1)−(the prediction mode of the leftblock−1)−(the prediction mode of the top block+1).

Validation may be performed on the candidate group configuration, and ifit is valid, it is included in the candidate group and if it is notvalid, it goes to the next candidate. If a neighboring block is locatedoutside of a picture or belongs to a different division unit from acurrent block, or if a coding mode of the corresponding block is interprediction, it may not be valid. In addition, it may not be valid evenif it is not possible to refer to it (described later in the presentinvention)

In the case of a spatially adjacent block among the candidates, it maybe composed of one block, or may be composed of several blocks (subblocks). Therefore, in the same order as (left—top) of the candidategroup configuration, the left block may be validated at any one position(for example, the bottom block among the left blocks) and then the ordermay be shifted to the top block, may be validate at several positions(for example, one or more sub-blocks starting downward from the topblock among the left block) and then the order may be shifted to the topblock, or may be decided depending on an encoding setting.

A moving motion model and a non-moving motion model may be classified inthe inter prediction unit according to a motion prediction method. Inthe case of the moving motion model, prediction may be performed byconsidering only a parallel movement, and in the case of the non-movingmotion model, prediction may be performed by considering not only theparallel movement but also movements such as rotation, perspective, andzoom in/out. Assuming a unidirectional prediction, one motion vector maybe required for the moving motion model, but one or more motion vectorsmay be required for the non-moving motion model. In the case of thenon-moving motion model, each motion vector may be information appliedto a predetermined position of the current block, such as a top leftvertex and an top right vertex of the current block, and a position of aarea to be predicted of the current block may be acquired in a pixelunit or a sub block unit through the corresponding motion vector. Theinter prediction unit may be commonly applied to some processesdescribed below according to the motion model, and some processes may beindividually applied.

The inter prediction unit may include a reference picture configurationunit, a motion estimation unit, a motion compensation unit, a motioninformation determination unit, and a motion information encoding unit.In the reference picture configuration unit, pictures that are encodedbefore or after the current picture may be included in the referencepicture lists L0 and L1. A prediction block may be obtained from areference picture included in the reference picture list, and thecurrent picture may also be configured as a reference picture andincluded in at least one of the reference picture lists according to anencoding setting.

In the inter prediction unit, the reference picture configuration unitmay include a reference picture interpolation unit, and may perform aninterpolation process for a fractional pixel according to interpolationprecision. For example, an 8-tap DCT based interpolation filter may beapplied for a luminance component, and a 4-tap DCT based interpolationfilter may be applied for a chrominance component.

In the inter prediction unit, the motion estimation unit searches ablock having a high correlation with the current block through areference picture, and various methods such as a full search-based blockmatching algorithm (FBMA) and a three step search (TSS) may be used. Inaddition, the motion compensation unit means a process of obtaining aprediction block through a motion estimation process.

In the inter prediction unit, the motion information determination unitmay perform a process for selecting the optimal motion information ofthe current block, and the motion information may be encoded by a motioninformation encoding mode such as a skip mode, a merge mode, or acompetition mode. The mode may be configured by combining supportedmodes according to a motion model. A skip mode (moving), a skip mode(non-moving), a merge mode (moving), a merge mode (non-moving), acompetition mode (moving), and a competitive mode (non-moving) may be anexample of that. Some of the modes may be included in the candidategroup according to an encoding setting.

In the motion information encoding mode, a prediction value of motioninformation (motion vector, reference picture, prediction direction,etc.) of the current block may be obtained from at least one candidateblock, and optimal candidate selection information may occur when two ormore candidate blocks are supported. In the skip mode (no residualsignal) and the merge mode (the residual signal present), the predictionvalue may be used as the motion information of the current block, and inthe contention mode, difference information between the motioninformation of the current block and the prediction value may begenerated.

A candidate group for a prediction value of the motion information of acurrent block may be adaptive and have various configurations accordingto a motion information encoding mode. Motion information of a blockspatially adjacent to the current block (e.g., left, top, top left, topright, bottom left block, etc.) may be included in the candidate group.In addition, motion information of temporally adjacent blocks (e.g.,left, right, top bottom, top right, left, bottom, bottom, and the like,including blocks in the other image corresponding to or corresponding tothe current block <center>) may be included to the candidate group. Inaddition, mixed motion information of a spatial candidate and a temporalcandidate (for example, information obtained by averaging, a medianvalue, etc. through motion information of a spatially adjacent block andmotion information of a temporally adjacent block, the motioninformation may be included on a current block or in a sub-block unit ofthe current block) may be included in the candidate group.

There may be a priority for constructing a prediction value candidategroup of the motion information. The order of inclusion in theconfiguration of the prediction value candidate group may be determinedaccording to a priority, and if the number of the prediction valuecandidates is determined according to the priority (determined accordingto an encoding mode of motion information), the configuration of theprediction value candidate group may be completed. In this case, thepriority may be determined in order of motion information of spatiallyadjacent blocks, motion information of temporally adjacent blocks, andmixed motion information of spatial candidates and temporal candidates,but other modifications are possible.

For example, spatially adjacent blocks may be included in the candidategroup in the order of left—top—top right—bottom left—top left block, andso on, and temporally adjacent blocks may be included in the order ofbottom right—middle—right—bottom block.

Validation may be performed on the candidate group configuration, and ifit is valid, it is included in the candidate group and if it is notvalid, it goes to the next candidate. If a neighboring block is locatedoutside of a picture or belongs to a different division unit from acurrent block, or if a coding mode of the corresponding block is interprediction, it may not be valid. In addition, it may not be valid evenif it is not possible to refer to it (described later in the presentinvention)

Adjacent blocks in terms of spatial or temporality among the candidates,may be composed of one block, or may be composed of several blocks (subblocks). Therefore, in the same order as (left-top) of a configurationof a spatial candidate group, the left block is validated at any oneposition (for example, the bottom block among the left blocks), and thenthe order is shifted to the top block, or the left block is validated atseveral positions (e.g., one or more sub-blocks located in the downwarddirection starting from the top block among the left blocks) and thenthe order may be shifted to top block. In addition, in the same order as(middle-right) of a configuration of a temporal candidate group, themiddle block may be validated at any one position and then the order maybe shifted to the right block. Alternatively, the middle block may bevalidated at several positions (e.g., one or more sub-blocks startingfrom a predetermined position block <2,2> and located in a predeterminedorder such as <3,3>, <2,3>, etc.) and then the order may be shifted tothe bottom block, or may be decided depending on an encoding setting.

The subtractor 205 subtracts a prediction block from a current block togenerate a residual block. That is, the subtractor 205 calculates adifference between a pixel value of each pixel of the current block tobe encoded and a predicted pixel value of each pixel of a predictionblock generated by the prediction unit to generate a residual block thatis a residual signal in a form of a block.

The converter 210 converts a residual block into a frequency domain toconvert each pixel value of the residual block into a frequencycoefficient. Herein, in the transform unit 210, the residual signal maybe transformed into the frequency domain using various transformationtechniques for transforming an image signal of a spatial axis into afrequency axis such as a Hadamard transform, a discrete cosine transformbased transform (DCT Based Transform), a discrete sine transform basedtransform (DST Based Transform), the Karunen Rube transform basedtransform (KLT Based), and the residual signal transformed into thefrequency domain becomes a frequency coefficient. The transform can betransformed by a one-dimensional transform matrix. Each transformationmatrix may be adaptively used in horizontal and vertical units. Forexample, in the case of intra prediction, when the prediction mode ishorizontal, a DCT based transform matrix may be used in the verticaldirection, and a DST based transform matrix may be used in thehorizontal direction. When the prediction mode is vertical, a DCT basedtransformation matrix may be used in the horizontal direction, and a DSTbased transformation matrix may be used in the vertical direction.

The quantization unit 215 quantizes a residual block having thefrequency coefficient transformed by the transformation unit 210 into afrequency domain. Herein, the quantization unit 215 may quantize thetransformed residual block using dead zone uniform thresholdquantization, a quantization weighted matrix, or an improvedquantization technique. This may include one or more quantizationtechniques as candidates and may be determined by a coding mode,prediction mode information, and the like.

The entropy encoding 245 scans the generated quantization frequencycoefficient sequence according to various scan methods to generate aquantization coefficient sequence, and outputs the encoded quantizationcoefficient sequence by using an entropy encoding technique, etc. Thescan pattern may be set to one of various patterns such as zigzag,diagonal lines, and rasters. In addition, encoded data including encodedinformation transmitted from each component may be generated and outputas a bitstream.

The inverse quantization unit 220 inversely quantizes the residual blockquantized by the quantization unit 215. That is, the inversequantization unit 220 inversely quantizes the quantized frequencycoefficient sequence to generate a residual block having the frequencycoefficient.

The inverse transform unit 225 inversely transforms the residual blockinversely quantized by the inverse quantization unit 220. That is, theinverse transformation unit 225 inversely transforms frequencycoefficients of the inversely quantized residual block to generate aresidual block having a pixel value, that is, a reconstructed residualblock. Here, the inverse transform unit 225 may perform inversetransform by using a transformation method used in the transform unit210 as the inverse.

The adder 230 reconstructs the current block by adding the predictionblock predicted by the prediction unit 200 and the residual blockreconstructed by the inverse transform unit 225. The reconstructedcurrent block may be stored in the decoded picture buffer 240 as areference picture (or a reference block) and used as a reference picturewhen encoding the next block of the current block, another block in thefuture, or another picture.

The filter unit 235 may include one or more post-processing filterprocesses such as a deblocking filter, a sample adaptive offset (SAO),an adaptive loop filter (ALF), and the like. The deblocking filter mayremove block distortion generated at the boundary between blocks in thereconstructed picture. The ALF may perform filtering based on a valueobtained by comparing the reconstructed image with the original imageafter the block is filtered through the deblocking filter. The SAOrestores an offset difference from the original image on the basis of apixel with respect to the residual block to which the deblocking filteris applied, and may be applied in the form of a band offset or an edgeoffset. Such a post-processing filter may be applied to thereconstructed picture or block.

The deblocking filter in the filter unit may be applied based on a pixelincluded in several columns or rows included in both blocks based on ablock boundary. The block may be applied to a boundary of an encodingblock, a prediction block, and a transform block, and may be limited toblocks having a predetermined minimum size (for example, 8×8).

In filtering, whether or not filtering is applied and filtering strengthmay be determined in consideration of characteristics of the blockboundary, and may be determined as one of candidates such as strongfiltering, intermediate filtering, and weak filtering. In addition, whena block boundary corresponds to a boundary of a division unit, it isdetermined whether to apply an in-loop filter at the boundary of thedivision unit.

In the filter unit, SAO may be applied based on a difference between thereconstructed image and the original image. An edge offset and a bandoffset may be supported as the type of offset, and one of the offsetsmay be selected according to an image characteristic to performfiltering. In addition, the offset-related information may be encoded onthe basis of a block and may be encoded through a prediction valuethereof. In this case, relevant information may be adaptively encodedaccording to the case where the prediction value is correct or the casewhere the prediction value is not correct. The prediction value may beoffset information of an adjacent block (e.g., left, top, top left, topright block, etc.), and selection information on which block to obtainoffset information may be generated.

Validation may be performed on the candidate group configuration, and ifit is valid, it is included in the candidate group and if it is notvalid, it goes to the next candidate. If a neighboring block is locatedoutside of a picture or belongs to a different division unit from acurrent block, it may not be valid. In addition, it may not be valideven if it is not possible to refer to it (described later in thepresent invention)

The decoded picture buffer 240 may store a reconstructed block orpicture by the filter unit 235. The reconstructed block or picturestored in the decoded picture buffer 240 may be provided to theprediction unit 200 that performs intra prediction or inter prediction.

FIG. 3 is a block diagram of an image decoding apparatus according to anembodiment of the present invention.

Referring to FIG. 3, the image decoding apparatus 30 may include anencoded picture buffer 300, an entropy decoding unit 305, a predictionunit 310, an inverse quantization unit 315, an inverse transformationunit 320, and an adder/subtractor 325, a filter 330, and a decodedpicture buffer 335.

In addition, the prediction unit 310 may be configured to include anintra prediction module and an inter prediction module.

First, when an image bitstream transmitted from the image encodingapparatus 20 is received, the image bitstream may be stored in theencoded picture buffer 300.

The entropy decoding unit 305 may decode the bitstream to generatequantized coefficients, motion vectors, and other syntax. The generateddata may be transferred to the prediction unit 310.

The prediction unit 310 may generate a prediction block based on thedata transferred from the entropy decoding unit 305. In this case, thereference picture list using a default construction technique may beconstructed based on the reference picture stored in the decoded picturebuffer 335.

The intra prediction unit may include a reference pixel configurationunit, a reference pixel filter unit, a reference pixel interpolationunit, a prediction block generation unit, and a prediction mode decodingunit. In addition, the inter prediction unit may include a referencepicture configuration unit, a motion compensation unit, and a motioninformation decoding unit, and some of the processes may be performed inthe same manner as an encoder and some may be reversed.

The inverse quantization unit 315 may inversely quantize the quantizedtransform coefficients provided in the bitstream and decoded by theentropy decoding unit 305.

The inverse transform unit 320 may generate a residual block by applyinginverse transform techniques of inverse DCT, inverse integer transforms,or the like to a transform coefficient.

In this case, the inverse quantization unit 315 and the inversetransform unit 320 may perform inversely the processes performed by thetransform unit 210 and the quantization unit 215 of the image encodingapparatus 20 described above, and may be implemented in various ways.For example, the same process and inverse transform shared with thetransform unit 210 and the quantization unit 215 may be used, andinformation about the transform and quantization process from the imageencoding apparatus 20 (e.g., transform size, transform shape,quantization type, etc.) to reverse the transform and quantizationprocesses.

The residual block that has undergone inverse quantization and inversetransformation may be added to the prediction block derived by theprediction unit 310 to generate an image block reconstructed. Thisaddition can be made by the adder and subtractor 325.

The filter 330 may apply a deblocking filter to the reconstructed imageblock to remove blocking if necessary, and may further use other loopfilters to improve video quality before and after the decoding process.

The image block that has been reconstructed and filtered may be storedin the decoded picture buffer 335.

Although not shown in the drawing, the image decoding apparatus 30 mayfurther include a division unit. In this case, the division unit mayinclude a picture division unit and a block division unit. The divisionunit may be easily understood by a person skilled in the art in the sameor corresponding configuration as that of an image encoding apparatus ofFIG. 2, and thus a detailed description thereof will be omitted.

FIGS. 4a to 4d are conceptual diagrams for illustrating a projectionformat according to an embodiment of the present invention.

FIG. 4a illustrates an ERP (Equi-Rectangular Projection) format in whicha 360-degree image is projected onto a two-dimensional plane. FIG. 4billustrates a CMP (CubeMap Projection) format in which a 360-degreeimage is projected onto a cube. FIG. 4c illustrates an OHP (OctaHedronProjection) format in which a 360-degree image is projected on anoctahedron. FIG. 4d illustrates an ISP (IcoSahedral Projection) formatin which a 360-degree image is projected on a polyhedron. However, thepresent invention is not limited thereto, and various projection formatsmay be used. For example, Trunked Square Pyramid Projection (TSP),Segmented Sphere Projection (SSP), and the like may also be used. FIGS.4a to 4d illustrates an example in which a left side is converted into a3D model and a right side is converted into a 2D space through aprojection process. The figure projected in two dimensions may becomposed of one or more surfaces, and each surface may have a shape of acircle, a triangle, a rectangle, or the like.

FIGS. 4a to 4d , the projection format may have one surface (e.g., ERP)or multiple surfaces (e.g., CMP, OHP, ISP, etc.). In addition, eachsurface may be divided into a square and a triangle. The division may bean example of the type, characteristic, and the like of the image in thepresent invention, which may be applied when the encoding/decodingsetting according to the projection format is different. For example,the type of the image may be a 360-degree image, and a characteristic ofthe image may be one of the above classifications (e.g., each projectionformat, a projection format of one surface or multiple surfaces, aprojection format in which the surface is rectangular or not), and thelike.

The two-dimensional plane coordinate system (for example, (i, j)) may bedefined on each surface of the two-dimensional projection image, and acharacteristic of the coordinate system may vary depending on theprojection format and the position of each surface. In the case of ERP,one two-dimensional planar coordinate system and other projectionformats may have multiple two-dimensional planar coordinate systemsaccording to the number of surfaces. In this case, the coordinate systemmay be represented by (k, i, j), where k may be index information ofeach surface.

In the present invention, for convenience of description, a descriptionwill be given focusing on the case where a surface is rectangular. Thenumber of surfaces projected in two dimensions may be one (e.g.,Equirectangular Projection, that is, the case of that the image is thesame as one surface) to two or more (e.g., Cube Map Projection, etc.).

FIGS. 5a to 5c are conceptual diagrams for illustrating a surfacearrangement according to an embodiment of the present invention.

In a projection format in which a three-dimensional image is projectedin two dimensions, it is necessary to determine a surface arrangement.In this case, the surface arrangement may be arranged to maintain imagecontinuity in three-dimensional space, and even if the image continuitybetween some adjacent surfaces is impaired, the surface may be arrangedas closely as possible. In addition, when placing the surface, some ofthe surface may be arranged by rotating at a certain angle (0, 90, 180,270 degrees, etc.).

Referring to FIG. 5a , an example of a surface arrangement for a CMPformat may be seen. When arranged to maintain image continuity inthree-dimensional space, a 4×3 layout may be used in which four surfacesare arranged horizontally and one surface is arranged up and down asshown in the left figure. In addition, as shown in the figure on theright, even if the image continuity between some adjacent surfaces isdamaged, a 3×2 layout in which the surfaces are arranged so that thereis no empty space in the two-dimensional plane may be used.

Referring to FIG. 5b , a surface arrangement of the OHP format may beconfirmed. When arranged to maintain image continuity in thethree-dimensional space, it may be as shown in the upper picture. Inthis case, even if the image continuity is partially impaired, thesurface may be arranged as shown in the figure below so that there is noempty space in the projected two-dimensional space.

Referring to FIG. 5c , it may be seen that a surface arrangement for theISP format. As shown in the figure above, it can be arranged to maintainimage continuity in three-dimensional space, and as shown in the figurebelow, it can be arranged so that there is no empty space between thesurfaces.

In this case, a process of arranging surfaces in close contact with eachother so that there is no empty space may be referred to as framepacking. By rotating the surface, image continuity may be reduced asmuch as possible. In the following, changing the surface arrangementdescribed above to another surface arrangement is referred to as surfacerelocation.

In the following description, the term continuity may be interpreted torefer to continuity of a scene that is visible in three-dimensionalspace, or continuity of an actual image or scene in a two-dimensionalprojected space. The existence of continuity may be expressed as a highcorrelation between areas. In a typical two-dimensional image, acorrelation between areas may be high or low, but a 360-degree image mayhave a area where continuity does not exist even though it is spatiallyadjacent. In addition, there may be areas where continuity is presentbut not spatially adjacent, depending on the preceding surfacearrangement or relocation.

Surface relocation may be performed for the purpose of improvingencoding performance. For example, a surface may be rearranged such thatsurfaces with image continuity are disposed adjacent to each other.

In this case, a surface relocation does not necessarily meanreconstruction after the surface arrangement, but may be understood as aprocess of setting a specific surface arrangement from the beginning.(may be done in area-wise packing of a 360-degree imageencoding/decoding process)

In addition, a surface arrangement or relocation may include rotation ofthe surface as well as changing a position of each surface (simplemovement of the surface in this example, such as moving from the topleft to the bottom left or the bottom right of the image). Herein, therotation of the surface may be expressed as 0 degrees, 45 degrees to theright, 90 degrees to the left, etc., if there is no surface rotation,and the angle of rotation may be referred to by dividing 360-degreesinto k (or 2k) sections (even or unequal) and selecting the dividedsections.

An encoder/decoder, according to preset surface arrangement information(surface shape, number of surfaces, position of the surface, angle ofrotation of the surface, etc.) and/or surface relocation information(position of each surface or angle to be moved, direction of movement,etc.), may perform a surface arrangement or relocation. In addition, theencoder may generate surface arrangement information and/or surfacerelocation information according to the input image, and the decoder mayperform surface arrangement (or relocation) by receiving and decodingthe information from the encoder.

In the following, when referring to a surface without a separatedescription, it is assumed on the basis of the 3λ2 layout (layout)according to FIG. 5a , the number indicating each surface may be from 0to 5 in a raster scan order from the top left.

In the following description on the premise of continuity betweensurfaces according to FIG. 5a , unless otherwise stated, surfaces 0through 2 may be assumed to have continuity with each other, surfaces 3through 5 may be assumed to have continuity with each other, andsurfaces 0 and 3, 1 and 4, and 2 and 5 may be assumed to have nocontinuity. The presence or absence of continuity between the abovesurfaces can be confirmed by setting of a characteristic, type, andformat of an image, etc.

According to an encoding/decoding process of a 360-degree image, anencoding apparatus may acquire an input image, perform preprocessing onthe acquired image, perform encoding on the preprocessed image, andtransmit the encoded bitstream to the decoding apparatus. Herein, thepreprocessing may include image stitching, projection of athree-dimensional image to a two-dimensional space, surface arrangementand relocation (or may be referred to as area-wise packing), and thelike. In addition, the decoding apparatus may generate an output imageby receiving the bitstream, decoding the received bitstream, andperforming post-processing (image rendering, etc.) on the decoded image.

In this case, the bitstream may include information (SEI message ormetadata) generated in the preprocessing process and information (imageencoded data) generated in the encoding process.

FIGS. 6a and 6b are exemplary diagrams for illustrating a division unitaccording to an exemplary embodiment of the present invention.

The image encoding/decoding apparatus according to FIG. 2 or FIG. 3 mayfurther include a division unit, and the division unit may include apicture division unit and a block division unit. The picture divisionunit may partition a picture into at least one processing unit {forexample, color space (YCbCr, RGB, XYZ, etc.), subpicture, slice, tile,basic coding unit (or maximum coding unit), etc.}, and the blockdivision unit may partition the basic coding unit into at least oneprocessing unit (e.g., encoding, prediction, transformation,quantization, entropy, in-loop filter unit, etc.).

The basic coding unit may be obtained by dividing a picture at regularintervals in a horizontal direction and a vertical direction, and may bea unit applied to a subpicture, a tile, a slice, a surface, and thelike. That is, the unit may be configured as an integer multiple of thebasic coding unit, but is not limited thereto.

For example, a size of a basic coding unit may be differently applied tosome division units (tile, subpicture, etc.), and the correspondingdivision unit may have a size of an individual basic coding unit. Thatis, the basic coding unit of the division unit may be set to be the sameas or different from the basic coding unit of the division unitdifferent from a basic coding unit of a picture unit.

For convenience of description, in the present invention, a basic codingunit and other processing units (encoding, prediction, transformation,etc.) are referred to as a block.

The size or shape of the block may be N×N square form in which a lengthand width are represented by an exponential power of 2n (2n×2n. 256×256,128×128, 64×64, 32×32, 16×16, 8×8, 4×4, etc., n is an integer between 2and 8), or M×N rectangular form (2m×2n). For example, in the case of ahigh resolution 8k UHD image, a 256×256 1080p HD image, or a 128×128WVGA image, an input image may be divided into 16×16 sizes and the like.

A picture may be divided into at least one slice. In the case of aslice, the slice may consist of a bundle of at least one consecutiveblock according to a scan pattern. Each slice may be divided into atleast one slice segment, and each slice segment may be divided intobasic coding units.

A picture may be divided into at least one subpicture or tile. Thesubpicture or tile may have a division form of a rectangle (rectangularor square), and may be divided into basic coding units. The subpictureis similar to the tile in that they have the same division form(squares). However, the subpicture may be distinguished from the tile inthat the subpicture has a separate encoding/decoding setting unlike thetile. That is, while the tile is assigned setting information forperforming encoding/decoding in a higher unit (eg, a picture, etc.), thesubpicture may directly obtain at least one setting information forperforming encoding/decoding from header information of each subpicture.That is, unlike the subpicture, the tile is only a unit obtainedaccording to division of an image and may not be a unit for transmittingdata (for example, a basic unit of a video coding layer (VCL)).

In addition, the tile may be a division unit supported from a parallelprocessing point of view, and the subpicture may be a division unitsupported from an individual encoding/decoding point of view. In detail,in the case of the subpicture, it may mean that not only theencoding/decoding setting may be set on the basis of a subpicture, butalso whether or not the encoding/decoding is determined. Alternatively,it may mean that a display is possible by configuring the correspondingsubpicture around an area of interest. Settings for this may bedetermined on the basis of a sequence, picture, and the like.

In the above example, it is also possible to change to include anencoding/decoding setting in a higher unit as a subpicture and to set anindividual encoding/decoding setting in a tile unit. In the presentinvention, for convenience of description, it is assumed that the tilemay be individually set or higher unit dependent setting.

Division information generated when a picture is divided intoquadrangular forms may have various forms.

Referring to FIG. 6a , a quadrangular division unit may be obtained bydividing a picture into a horizontal line b7 and a vertical line (inthis case, b1 and b3, b2 and b4 each become one dividing line). Numberinformation of the rectangles may be generated based on the horizontaland vertical directions, respectively. In this case, if the rectangle isequally divided, the horizontal and vertical length of the dividedrectangle by dividing the width and height of the picture by the numberof horizontal and vertical lines may be checked, and if the rectangle isnot evenly divided, information indicating the width and length of therectangle may be additionally generated. In this case, the horizontaland vertical lengths may be expressed in one pixel unit or in multiplepixel units. For example, when expressed in multiple pixel units, if abasic coding unit is M×N and a rectangle is 8M×4N, the horizontal andvertical lengths may be expressed as 8 and 4 (when the basic coding unitof the corresponding division unit is M×N in this example), or 16 and 8(when the basic coding unit of the corresponding division unit isM/2×N/2).

Meanwhile, referring to FIG. 6b , unlike FIG. 6a , a case where apicture division unit is obtained by separately dividing a picture maybe confirmed. For example, the number of rectangles in an image,horizontal and vertical start position information of each rectangle (asindicated by the drawing symbols z0 to z5, may be represented by the xand y coordinates in the picture) and horizontal and vertical lengthinformation of each rectangle may be generated. In this case, the startposition may be expressed in one pixel unit or multiple pixel units, andthe horizontal and vertical lengths may also be expressed in one pixelunit or multiple pixel units.

FIG. 6a may be an example of division information of a tile or a subpicture, and FIG. 6b may be an example of division information of a subpicture, but is not limited thereto. Hereinafter, for convenience ofdescription, a tile is assumed to be a rectangular division unit, butthe tile-related description may be applied to the subpicture in thesame or similar manner (also, it may be applied to a surface). That is,in the present invention, only the difference in terms is used, meaningthat a description of a tile may be used as a definition of a subpictureor a description of a subpicture may be used as a definition of a tile.

Some of the above-mentioned division units may not necessarily beincluded, all or some of them may be selectively included depending onan encoding/decoding setting, and another additional unit (e.g.,surfaces) may be supported.

Meanwhile, the block partition unit may be partitioned into coding units(or blocks) having various sizes. In this case, the coding unit may beconfigured of multiple coding blocks (e.g., one luminance coding block,two color difference coding blocks, etc.) according to a color format.For convenience of explanation, it is assumed that one color componentunit is used. The coding block may have a variable size such as M×M(e.g., M is 4, 8, 16, 32, 64, 128, etc.). Alternatively, according tothe division scheme (for example, tree-based division, quad treedivision <Quad Tree. QT>, binary tree division <Binary Tree. BT>,ternary tree division <Ternary Tree. TT>, etc.), the coding block mayhave a variable size such as M×N (e.g., M and N are 4, 8, 16, 32, 64,128, etc.). In this case, the coding block may be a unit that is thebasis of intra prediction, inter prediction, transformation,quantization, entropy encoding, and the like.

In the present invention, the description will be made under theassumption that multiple sub-blocks (symmetry) having the same size andshape are obtained according to a division scheme, but application to acase having an asymmetric sub-block (e.g., For binary trees, thehorizontal ratio between division blocks is equal to 1:3 or 3:1<vertical is the same>, or the vertical ratio is equal to 1:3 or 3:1<horizontal is the same>. For ternary trees, the horizontal ratiobetween division blocks is equal to 1:2:1<vertical is the same>, or thevertical ratio is equal to 1:2:1 <horizontal is the same>) may also bepossible.

A division of a coding block M×N may have a recursive tree-basedstructure. In this case, whether to divide may be indicated by adivision flag. For example, when the division flag of the coding blockhaving the division depth k is 0, encoding of the coding block isperformed in the coding block having the division depth k, and when thedivision flag of the coding block having the division depth k is 1, theencoding of the coding block is performed in four sub-coding blocks(quad tree division), two sub-coding blocks (binary tree division), orthree sub-coding blocks (ternary tree division) having the divisiondepth of k+1 according to the division scheme.

The sub-coding block may be set as a coding block k+1 again and dividedinto sub-coding blocks k+2 through the above process. In the case ofquad-tree division, a division flag (for example, indicating whether todivide or not) may be supported.

In the case of binary tree division, a division flag and a divisiondirection flag (horizontal or vertical) may be supported. If in binarytree division, one or more division ratios (e.g., additional divisionratio with aspect ratio other than 1:1. I.e. asymmetric division is alsosupported) are supported, a division ratio flag (e.g., select a ratio ofone of the aspect ratio candidates <1:1, 1:2, 2:1, 1:3, 3:1>) may besupported or another type of flag (e.g., whether to dividesymmetrically. 1 means symmetric division, no additional information,and 0 means asymmetric division, additional information about ratio isrequired.) may be supported.

In the case of ternary tree division, a division flag and a divisiondirection flag may be supported. If the ternary tree division supportsmore than one division ratio, additional division information such asthe binary tree may be needed.

The above example is division information generated when only one treedivision is valid, and when multiple tree divisions are valid, divisioninformation may be configured as follows.

For example, when multiple tree divisions are supported, when there is apreset division priority, division information corresponding to thepriority may be configured first. In this case, when the division flagcorresponding to the prior rank is true (performing division),additional division information of a division method may be continued,and when the division flag is false (performing division x), divisioninformation of the division method corresponding to a later rank may beconfigured.

Alternatively, when multiple tree divisions are supported, selectioninformation on a division method may additionally occur, and may beconfigured as division information according to the selected divisionmethod.

The division flag may be omitted according to a preceding higher orprevious division result.

A block division may start from the largest coding block and proceed tothe minimum coding block. Alternatively, it may start from the minimumdivision depth 0 and proceed to the maximum division depth. That is,division may be performed recursively until a block size reaches theminimum coding block size or the division depth reaches the maximumdivision depth. In this case, a size of the maximum coding block, a sizeof the minimum coding block, and the maximum division depth may beadaptively set according to an encoding/decoding setting (e.g., image<slice, tile> type <I/P/B>, encoding mode <Intra/Inter>, colordifference component <Y/Cb/Cr>, etc.)

For example, when the maximum coding block is 128×128, quad treedivision may be performed in a range of 32×32 to 128×128, binary treedivision may be performed in a range of 16×16 to 64×64 and the maximumdivision depth of 3, and ternary tree division may be performed in arange of 8×8 to 32×32 and a maximum division depth of 3. Alternatively,quad tree division may be performed in a range of 8×8 to 128×128, andbinary tree and ternary tree division may be performed in a range of 4×4to 128×128 and a maximum division depth of 3. In the former case, it maybe the setting in an I image type (for example, a slice), and in thelatter case, it may be the setting in a P or B image type.

As described in the above example, division settings such as the maximumcoding block size, the minimum coding block size, the maximum divisiondepth, and the like may be commonly or separately supported according tothe division method and the above-described encoding/decoding setting.

When multiple division schemes are supported, division is performedwithin a block support range of each division scheme, and when a blocksupport range of each division scheme overlaps, priority of the divisionscheme may exist. For example, quad tree division may precede binarytree division.

Alternatively, division selection information that is performed when thedivision support ranges overlap. For example, selection information on adivision method performed during binary tree and ternary tree divisionmay occur.

In addition, when multiple division schemes are supported, it may bedetermined whether to perform a subsequent division according to aresult of a preceding division. For example, when the result of thepreceding division (quad tree in this example) indicates that divisionis performed, the subsequent division (binary tree or ternary tree inthis example) is not performed, and the sub-coding block dividedaccording to the preceding division may be set again as a coding blockto perform division.

Alternatively, when the result of the preceding division indicates thatthe division is not performed, the division may be performed accordingto the result of the subsequent division. In this case, when the resultof the subsequent division (binary tree or ternary tree in this example)indicates that the division is to be performed, the divided sub-codingblock may be set again as a coding block to perform division, and whenthe result of the subsequent indicates that no division is to beperformed, no further division is performed. In this case, when thesubsequent division result indicates that the division is performed andthe divided sub-coding block is set again as a coding block, whenmultiple division schemes are supported (for example, the block supportrange of each division scheme overlaps), only the subsequent divisionmay be supported without performing the preceding division. That is,when multiple division schemes are supported, when the result of thepreceding division indicates that the division is not performed, itmeans that the previous division is not performed any more.

For example, when an M×N coding block is capable of quad tree divisionand binary tree division, a quad tree division flag may be checkedfirst. When the division flag is 1, division is performed into foursub-coding blocks having a size of (M>>1)×(N>>1), and the sub-codingblock is set as a coding block to perform division (quad tree divisionor binary tree division). When the division flag is 0, the binary treedivision flag may be checked, and when the binary tree division flag is1, division is performed into two sub-coding blocks having a size of(M>>1)×N or M×(N>>1), and the sub-coding block may be set as a codingblock again to perform division (binary tree division). When the binarytree division flag is 0, the division process is terminated and encodingis performed.

Although a case in which multiple division schemes are performed throughthe above example has been described, a combination of various divisionschemes may be possible without being limited thereto. For example, adivision scheme such as quad tree/binary tree/ternary tree/quadtree+binary tree/quad tree+binary tree+ternary tree may be used. In thiscase, information on whether an additional division scheme is supportedmay be implicitly determined or explicitly included in a unit such as asequence, a picture, a subpicture, a slice, a tile, and the like.

In the above example, information related to a division, such as sizeinformation of a coding block, a support range of the coding block, themaximum division depth, and the like, may be implicitly determined onthe basis of a sequence, a picture, a subpicture, a slice, a tile, andthe like. In summary, an allowable block range may be determined by asize of the largest coding block, a range of supported blocks, themaximum division depth, and the like.

The coding block obtained by performing the division through the aboveprocess may be set to the maximum size of intra prediction or interprediction. That is, the coding block after block division may be astart size of division of a prediction block for intra prediction orinter prediction. For example, when the coding block is 2M×2N, theprediction block may have sizes of 2M×2N and M×N smaller than or equalto that. Alternatively, the size may be 2M×2N, 2M×N, M×2N, or M×N.Alternatively, the size may be 2M×2N with the same size as the codingblock. In this case, when the coding block and the prediction block havethe same size may mean that the prediction is performed by a sizeobtained through the division of the coding block without dividing theprediction block. In other words, it means that division information forthe prediction block is not generated. Such a setting may also beapplied to a transform block, and the transform may be performed on thebasis of the divided coding block.

Various configurations may be possible according to the followingencoding/decoding settings. For example, at least one prediction blockand at least one transform block may be obtained based on a coding block(after the coding block is determined). Alternatively, one predictionblock having the same size as the coding block may be obtained, and atleast one transform block may be obtained based on the coding block.Alternatively, one prediction block and one transform block having thesame size as the coding block may be obtained. In the above example,when at least one block is obtained, division information of each blockmay be generated (or occur). When one block is obtained, divisioninformation of each block does not occur.

A block of square or rectangular shape of various sizes obtainedaccording to the result may be a block used for intra prediction andinter prediction, a block used for transforming or quantizing residualcomponents, or a block used in a filter process.

The division unit obtained by dividing a picture through the picturedivision unit may perform independent encoding/decoding or dependentencoding/decoding according to an encoding/decoding setting.

Independent encoding/decoding may mean that data of other units cannotbe referred to when performing encoding/decoding of some division units(or areas). In detail, information {e.g., pixel value orencoding/decoding information (intra-prediction related information,inter prediction related information, entropy encoding/decoding relatedinformation, etc.)} used or generated in a texture encoding and anentropy encoding in some units is independently encoded withoutreference to each other. In addition, in a decoder, parsing informationand reconstruction information of other units may not be referred toeach other for texture decoding and entropy decoding of some units.

In addition, dependent encoding/decoding may mean that data of otherunits may be referred to when performing encoding/decoding of someunits. In detail, information used or generated in a texture encodingand an entropy encoding in some units are mutually referred to andencoded, and in a decoder, parsing information and reconstructioninformation in other units may be used for a texture decoding and anentropy decoding in some units may be referenced to each other.

In general, the above-mentioned division unit (e.g., subpicture, tile,slice, etc.) may follow an independent encoding/decoding setting. Thiscan be set to be impossible to refer to for the purpose of parallelism.In addition, it may be set to be impossible to refer to for the purposeof improving an encoding/decoding performance. For example, when a360-degree image is divided into multiple surfaces in three-dimensionalspace and arranged in two-dimensional space, a correlation (for example,image continuity) with an adjacent surface may be degraded according tothe surface arrangement setting. In other words, if there is nocorrelation between the surfaces, since a need to refer to each other islow, an independent encoding/decoding setting can be followed.

In addition, a referenceable setting between division units may beprovided for the purpose of improving encoding/decoding performance. Forexample, even if an image is divided on the basis of a surface in a360-degree image, depending on the surface arrangement settings, theremay be a case where there is a high correlation with adjacent surfaces,and in this case, a dependent encoding/decoding setting may be followed.

Meanwhile, in the present invention, independent or dependentencoding/decoding may be applied not only to a spatial domain but alsoextended to a temporal domain. That is, not only to perform independentor dependent encoding/decoding with other division units existing withinthe same time as a current division unit, but also to performindependent or dependent encoding/decoding with division units existingwithin the different time from a current division unit (In this example,it is assumed to be the different division unit even if the divisionunit of the same position exists in an image corresponding to a timedifferent from the current division unit.) may be possible.

For example, when simultaneously transmitting a bitstream A containingdata encoded with a 360-degree image and a bitstream B containing dataencoded with a normal image quality, in a decoder, a area correspondingto a area of interest (e.g., an area where a user's eyes stay<viewport>, an area to display, or the like) may be parsed and decodedby the bitstream A transmitted with high image quality, and may bedecoded by parsing a bitstream B transmitted with normal image qualityother than the area of interest.

Specifically, when an image is divided into multiple units (for example,subpictures, tiles, slices, surfaces, etc., in this example, the surfaceis assumed to be data processed in the same way as a tile orsubpicture), it is possible to decode data (bitstream A) of the divisionunit belonging to the area of interest (or the division unit overlappingone pixel with the viewport) and data of the division unit belonging toa area other than the area of interest (bitstream B).

Alternatively, the bitstream including an encoded data of the entireimage may be transmitted, and the decoder may parse and decode the areaof interest from the bitstream. In detail, only data of a division unitbelonging to the area of interest may be decoded.

In summary, the encoder generates the bitstream divided by one or moreimage quality, and the decoder may decode only a specific bitstream toobtain all or part of the images, or selectively decode each bitstreamfor each part of the image to obtain all or part of the images. In theabove example, the case of a 360-degree image is taken as an example,but this may be a description applicable to a general image.

When performing encoding/decoding as in the above example, since adecoder does not know what data to restore (In this example, an encoderdoes not know where a area of interest is located. It is randomlyaccessed according to the area of interest), it is necessary to check areference setting in a time domain as well as a spatial domain toperform the encoding/decoding.

For example, in a situation where it is determined which decoding is tobe performed by a single division unit in a decoder, a current divisionunit may perform independent encoding in a spatial domain and limiteddependent encoding (e.g., reference is allowed only in the same positiondivision unit at a different time corresponding to the current divisionunit, other division units are limited in reference, and in the generalcase, there is no restriction in the time domain, so compared with thecase of unrestricted dependent encoding) in a time domain.

Alternatively, in a situation where it is determined which decoding(e.g., if one of the division units belongs to a area of interest,multi-unit decoding) is to be performed in multiple division units(multiple division units can be obtained by grouping horizontallyadjacent split units or by grouping vertically adjacent division units,or multiple division units can be obtained by grouping horizontally andvertically adjacent division units) in a decoder, the current divisionunit may perform independent or dependent encoding in the spatial domainand limited dependent encoding (e.g., allow referencing some otherdivision units as well as the same position division unit at a differenttime corresponding to the current division unit) in the time domain.

In the present invention, a surface is generally a dividing unit havinga different arrangement and shape according to a projection format, andhaving different characteristics from the other dividing units describedabove, such as not having a separate encoding/decoding setting. However,it may also be viewed as a unit obtained by a picture division unit interms of dividing an image into multiple areas (and having a rectangularshape, etc.).

In the case of a spatial domain, it has been described above thatindependent encoding/decoding can be performed for each division unitfor the purpose of parallelization or the like. However, independentencoding/decoding has a problem in that the encoding/decoding efficiencycan be reduced because other division units cannot be referred to.Therefore, as a step before the encoding/decoding is performed, thedivision unit in which the independent encoding/decoding is performedmay be extended by using (or adding) data of an adjacent division unit.Herein, since reference data is increased in the division unit to whichdata of the adjacent division unit is added, encoding/decodingefficiency may increase. In this case, encoding/decoding of the extendeddivision unit may also be regarded as dependent encoding/decoding interms of referencing the data of the adjacent division unit.

Information about a reference setting between division units may bestored in a bitstream on the basis of a video, a sequence, a picture, asubpicture, a slice, a tile, and the like and in the decoder, thesetting information transmitted from an encoder by parsing on the basisof the same level may be restored. In addition, related information maybe transmitted in a bitstream, and used by parsing in the form ofSupplement Enhancement Information (SEI) or Metadata. In addition,according to a pre-defined definition of the encoder/decoder, theencoder/decoder may be performed according to a reference settingwithout transmitting the information.

FIG. 7 is an exemplary diagram in which one picture is partitioned intomultiple tiles. FIGS. 8a to 8i are first exemplary diagrams for settingan additional area for each tile according to FIG. 7. FIGS. 9a to 9i aresecond exemplary diagrams for setting an additional area for each tileaccording to FIG. 7.

If an image is divided into two or more division units (or areas)through the picture division unit and encoding/decoding is independentlyperformed for each division unit, while there are advantages such asparallelism, data that can be referred to by each division unit isreduced, which may cause a problem of deterioration in encodingperformance. In order to solve this problem, it can be handled as adependent encoding/decoding setting between the division units (Thisexample is based on tiles. It can be applied to other units with thesame or similar settings).

In general, independent encoding/decoding proceeds to prevent referencebetween division units. Thus, pre- or post-processing for dependentencoding/decoding can be performed. For example, before performingencoding/decoding, an extended area may be placed outside of eachdivision unit, and the extended area may be filled with data of anotherdivision unit to be referred to.

This method does not differ in that it performs independentencoding/decoding except that each division unit is extended to performencoding/decoding, it can be understood as an example of dependentencoding/decoding in that data obtained by an existing division unit isreferred to in advance in another division unit.

In addition, after performing encoding/decoding, filtering may beapplied using multiple division unit datum based on a boundary betweendivision units. is, when the filtering is applied, different divisionunit data is used, and depending on the filtering, the filtering may beindependent when the filtering is not applied.

In the following example, a case of performing dependentencoding/decoding by performing encoding/decoding preprocessing(expansion in this example) will be described. In addition, in thepresent invention, a boundary between the same division units may bereferred to as an inner boundary, and an outline of the picture may bereferred to as an outer boundary.

According to an embodiment of the present invention, an additional areafor a current tile may be set. In detail, an additional area may be setbased on at least one tile (In this example, including when a pictureconsists of one tile. That is, even when the picture is not divided intotwo or more division units. Exactly, the division unit means that thedivision is divided into two or more units, but the case is alsorecognized as one division unit even if it is not divided).

For example, an additional area may be set in at least one of up, down,left, and right directions of a current tile. Herein, the additionalarea can be filled using an arbitrary value. In addition, the additionalarea may be filled using some data of the current tile, and it ispossible to pad the outer pixels of the current tile or to copy and fillthe pixels in the current tile.

In addition, the additional area may be filled using image data ofanother tile than the current tile. Specifically, image data of a tileadjacent to the current tile may be used, and it is possible to copy andfill image data of adjacent tiles in a specific direction amongup/down/left/right of the current tile.

A size (length) of the image data acquired here may have a value commonto each direction or may have an individual value, and it may bedetermined according to an encoding/decoding setting.

For example, in FIG. 6a , all or some of the boundaries between b0 andb8 may be extended. In addition, it may be extended by m in all boundarydirections of a division unit or may be extended by mi (i is eachdirection index) according to the boundary direction. The m or mi may beapplied to all division units of an image or may be individually setaccording to a division unit.

Herein, setting information relating to the additional area may begenerated. At this time, setting information of the additional areaincludes whether the additional area is supported, whether theadditional area is supported for each division unit, a form of theadditional area in an entire image (e.g., determined depend on whichdirection it extend among up, down, left, or right of the division unit.This example is setting information commonly applied to all divisionunits in an image), a form of the additional area in each division unit(This example is setting information that is applied to individualdivision units in an image), a size of the additional area in the entireimage (e.g., if the shape of the additional area is determined, it meanshow much to expand in the direction of expansion. This example issetting information commonly applied to all division units in an image),a size of the additional area in each division unit (This example issetting information that is applied to individual division units in theimage), a method of filling the additional area in the entire image, anda method of filling the additional area in each division unit.

The additional area related setting may be proportionally determinedaccording to a color format, or may have an independent setting. Thesetting information of the additional area in a luminance component canbe generated, and the setting of the additional area in a chrominancecomponent can be implicitly determined according to the color format.Alternatively, the setting information of the additional area in thechrominance component may be generated.

For example, when the size of the additional area of the luminancecomponent is m, the size of the additional area of the chrominancecomponent may be determined to be m/2 according to the color format(4:2:0 in this example). As another example, when the size of theadditional area of the luminance component is m and the chrominancecomponent has an independent setting, the size information of theadditional area of the chrominance component (n. In this example, n iscommonly used, or n1, n2, n3, etc. are possible depending on thedirection or extension area) may be generated. As another example, themethod of filling the additional area of the luminance component may begenerated, and the method of filling an additional area of thechrominance component may use the method in the luminance component orrelated information may be generated.

Information related to the setting of the additional area may berecorded and transmitted in a bitstream on the basis of a video, asequence, a picture, a subpicture, a slice, etc., and in decoding, therelevant information may be parsed and restored from the unit. Thefollowing embodiment will be described under the assumption thatadditional area support is activated.

Referring to FIG. 7, it may be seen that one picture is divided intorespective tiles displayed from 0 to 8. At this time, when an additionalarea according to an embodiment of the present invention is set for eachtile according to FIG. 7, it is the same as in FIGS. 8a to 8i

In FIGS. 7 and 8 a, tile 0 (the size of T0_W×T0_H) may be extended tofurther have an area of EOR to the right and E0_D to the bottom. In thiscase, an additional area may be obtained from an adjacent tile. In moredetail, the right extended area may be obtained from tile 1, and thebottom extended area may be obtained from tile 3. In addition, tile 0may set an additional area by using a tile adjacent to the bottom rightside (tile 4). That is, the additional area may be set in the directionof the inner boundary (or the boundary between the same division units)except for the outer boundary (or the picture boundary) of the tile.

In FIGS. 7 and 8 e, tile 4 (the size of T4 W×T4 H) may be extended tofurther include areas of left, right, top, and bottom since there is noouter boundary. In this case, the left extended area may be obtainedfrom tile 3, the right extended area of 5 tiles, the top extended areaof 1 tile, and the bottom extended area of 7 tile. In addition, tile 4can set an additional area such as top left, bottom left, top right, andbottom right. In this case, the top left extended area may be obtainedfrom tile 0, the bottom left extended area to tile 6, the top rightextended area to tile 2, and the bottom right extended area to beobtained from tile 8.

In FIG. 8c , since the L2 block is a block adjacent to the tile boundaryin principle, there is no data that can be referenced from the left, topleft, and bottom left blocks. However, when an additional area for tile2 is set according to an embodiment of the present invention, the L2block may be encoded/decoded with reference to the additional area. Thatis, the L2 block may refer to the data of the blocks located at the topleft and the left as an additional area (which may be the area obtainedfrom tile 1), and may refer to the data of the block located at thebottom left as an additional area (which may be the area obtained fromtile 4).

According to the above embodiment, data included in an additional areamay be included in a current tile to perform encoding/decoding. In thiscase, since the data of the additional area is located at a tileboundary (in this example, the tile is updated or expanded due to theadditional area), there is no data to refer to during an encodingprocess, so that encoding performance may be degraded, but since this isa portion added for a reference of an existing tile boundary area, Itcan be understood as a temporary memory format for improving encodingperformance. That is, since a area is helpful to improve the quality ofthe final output image and is eventually removed, the encodingperformance of the area is not deteriorated. This may be applied tosimilar or the same purpose in the embodiments described below.

Meanwhile, referring to FIGS. 9a to 9i , a 360-degree image is changedinto a 2D image through a surface arrangement (or rearrangement) processaccording to a projection format, and each tile (or may be a surface)obtained by dividing the 2D image may be checked. In this case, if the360-degree image is an Equirectangular, since the 2D image is composedof one surface, it may be an example of dividing one surface into tiles.In addition, for convenience of description, it is assumed that the tiledivision for the 2D image is the same as the tile division according toFIG. 7.

Herein, the divided tile may be divided into a tile consisting only ofan inner boundary and a tile including at least one outer boundary. Asin FIGS. 8a to 8i , an additional area may be set for each tile.However, the 360-degree image converted into the 2D image may not havecontinuity of an actual image even if adjacent to each other in the 2Dimage, or may have a continuity of the actual image even if not adjacent(referring to the description of FIGS. 5a to 5c ). Thus, even if someboundary of the tile is an outer boundary, an area in which continuityexists with the outer boundary area of the tile may exist in thepicture. Specifically, referring to FIG. 9b , the top part of tile 1corresponds to an outer boundary of the picture, but since an areahaving continuity of the actual image may exist in the same picture, itmay also be possible to set an additional area to the top of tile 1.That is, unlike FIGS. 8a to 8i , in FIGS. 9a to 9i , an additional areamay be set for all or part of the outer boundary direction of the tile.

Referring to FIG. 9e , tile 4 is a tile whose tile boundary consistsonly of an inner boundary (tile 4 in this example). Therefore, anadditional area for tile 4 may be set for all of the top, bottom, leftand right directions and the top left, bottom left, top right and bottomright directions. Here, the left extended area may be image dataobtained from tile 3, the right extended area may be image data obtainedfrom tile 5, the top extended area may be image data obtained from tile1, and the bottom extended area may be image data obtained from tiledata, the top left extended area may be image data obtained from tile 0,the bottom left extended area may be image data obtained from tile 6,the top right extended area may be image data obtained from tile 2, andthe bottom right extended area may be image data obtained from tile 8.

Referring to FIG. 9a , tile 0 corresponds to a tile having at least oneouter boundary (left and top directions). Accordingly, tile 0 may havean additional area extending not only in spatially adjacent right,bottom, and bottom right directions, but also in outer boundarydirections (left, top, and top left directions). Herein, an additionalarea may be set in spatially adjacent right, bottom, and bottom rightdirections using data of an adjacent tile, but an additional area withrespect to the outer boundary direction are problematic. Herein, theadditional area with respect to the outer boundary direction is notspatially adjacent in a picture, but may be set using data havingcontinuity in an actual image. For example, if the projection format ofa 360-degree image is Equirectangular, the left boundary of the picturehas substantially an image continuity with the right boundary of thepicture, and the top boundary of the picture has substantially the imagecontinuity with the bottom boundary of the picture, the left boundary oftile 0 has a continuity with the right boundary of tile 2, and the topboundary of tile 0 has continuity with the bottom boundary of tile 6.Therefore, in tile 0, the left extended area may be obtained from tile2, the right extended area may be obtained from tile 1, the top extendedarea may be obtained from tile 6, and the bottom extended area may beobtained from tile 3. In addition, in tile 0, the top left extended areamay be obtained from tile 8, the bottom left extended area may beobtained from tile 5, the top right extended area may be obtained fromtile 7, and the bottom right extended area may be obtained from tile 4.

Since the L0 block of FIG. 9a is a block located at a tile boundary,data (a situation similar to U0) that may be referred to from the left,top left, bottom left, top, and right blocks may not exist. In thiscase, although not spatially adjacent in a 2D image, a block havingcontinuity in an actual image may exist in the 2D image (or picture).Therefore, if a projection format of a 360-degree image isEquirectangular as shown in the foregoing premise, and the left boundaryof the picture has substantially the continuity of the image with theright boundary of the picture, and the top boundary of the picture hassubstantially the continuity of the image with the lower boundary of thepicture, the left and lower left blocks of the L0 block may be obtainedfrom tile 2, the top left block of the L0 block may be obtained fromtile 8, and the top and top right blocks of the L0 block may be obtainedfrom tile 6.

Table 1 below is a pseudo code for obtaining data corresponding to anadditional area from another area having continuity.

i_pos' = overlap (i_pos, minI, maxI) overlap (A, B, C) { if( A < B )output = (A + C − B + 1) % (C − B + 1) else if( A > C ) output = A % (C− B + 1) else output = A }

Referring to the pseudo code of Table 1, an overlap function parameteri_pos (corresponding to variable A) may mean an input pixel position,i_pos′ may mean an output pixel position, minI (corresponding tovariable B) may mean the minimum value of a pixel position range, andmaxI (corresponding to variable C) may mean a maximum value of the pixelposition range, i may mean a position component (horizontal, vertical,etc. in this example), and in this example, minI may be 0, and mail maybe Pic_width (picture width)−1 or Pic_height (picture height)−1.

For example, it is assumed that a vertical length of a picture (generalimage) has a range of 0 to 47 and the picture is divided as shown inFIG. 7. When the additional area is set to m below the tile 4 and theadditional area is filled using the data of the top part of the tile 7,the above equation may be used to determine where the data is obtained.

When the vertical length of the tile 4 ranges from 16 to 30 and hasadditional areas down by 4, the corresponding data of the positions 31,32, 33, and 34 may be filled into the additional area of the tile 4. Atthis time, since min and max are 0 and 47, respectively, 31 to 34 areoutputted as their own values of 31 to 34. That is, the data to befilled in the additional area is data of positions 31 to 34.

Alternatively, it is assumed that the horizontal length of a picture(360-degree image. Equirectangular. Both ends of the image havecontinuity) has a range of 0 to 95 and the picture is divided as shownin FIG. 7. When the additional area is set to m left the tile 3 and theadditional area is filled using the data of the right part of the tile5, the above equation can be used to determine where to obtain data.

When the width range of the tile 3 is 0 to 31 and has additional areasleft by 4, corresponding data of positions −4, −3, −2, and −1 may befilled into the additional area of the tile 3. Since the above positiondoes not exist in the horizontal length range of the picture, the aboveequation calculates where to obtain. At this time, since min and max are0 and 95, respectively, −4 to −1 are output as 92 to 95. That is, thedata to be filled in the additional area is data of positions 92 to 95.

In detail, when the area of m is data between 360 degrees and 380degrees (In this example, the pixel value position range is from 0 to360 degrees), it can be understood similarly to the case where the datais obtained in an area called 0 to 20 degrees by adjusting the internalrange of the image. That is, it can be obtained based on the pixel valueposition range of 0 to Pic_width−1.

In summary, in order to obtain data of an additional area, the positionof data to be acquired may be checked through an overlapping process.

In the above example, in the case of a 360-degree image in which onesurface is acquired, it is assumed that spatially adjacent areas withina picture have continuity with each other (except that a pictureboundary has continuity at both ends). However, if there are two or moresurfaces according to a projection format (for example, cube map, etc.),and each surface is arranged or rearranged, there may be a case wherethere is no continuity even though it is spatially adjacent in thepicture. In this case, the additional area may be generated by checkingthe data of the position having the continuity of an actual imagethrough the surface arrangement or relocation information.

Table 2 below is a pseudo code for generating an additional area for aspecific division unit by using internal data of the specific divisionunit.

i_pos' = clip (i_pos, minI, maxI) clip (A, B, C) { if( A < B ) output =B else if( A > C ) output = C else output = A }

The meanings of the variables according to Table 2 are the same as inTable 1, and thus detailed descriptions are omitted. In this example,minI may be the left or top coordinate of a specific division unit, andmaxI may be the right or bottom coordinate of each unit.

For example, when a picture is divided as shown in FIG. 7 and thehorizontal length of tile 2 is in the range of 32 to 47, and theadditional area is m to the right of tile 2, data corresponding to 48,49, 50, and 51 positions may be filled with data of 47 positions (insidetile 2) output through the above equation. That is, according to Table2, an additional area for a specific division unit may be generated bycopying the outer pixels of the division unit.

In summary, in order to obtain data of an additional area, a position tobe acquired may be checked through a clipping process.

The detailed configuration according to Table 1 or Table 2 is not fixedand can be changed. For example, in the case of a 360 degree image,overlapping may be changed and applied immediately in consideration ofthe arrangement (or relocation) of a surface and the coordinate systemcharacteristics between surfaces.

FIG. 10 is an exemplary diagram of using an additional area generatedaccording to an embodiment of the present invention in anencoding/decoding process of another area.

Meanwhile, since an additional area according to an exemplary embodimentof the present invention is generated using image data of another area,it may correspond to overlapped image data. Therefore, after theencoding/decoding is performed to prevent the maintenance of unnecessaryredundant data, the additional area may be removed. However, beforeremoving the additional area, it may be considered to remove theadditional area after using it in encoding/decoding.

Referring to FIG. 10, it may be seen that B, which is an additional areaof division unit J, is generated using area A of division unit I. Inthis case, before removing the generated area B, the area B may be usedfor encoding/decoding (in detail, a restoration or correction process)of the area A included in the division unit I.

Specifically, if the division unit I and the division unit J are assumedto be tiles 0 and 1 according to FIG. 7, the right part of the divisionunit I and the left part of the division unit J have image continuitywith each other. Herein, the additional area B may be used forencoding/decoding the area A after referring to the imageencoding/decoding of the division unit J. In particular, although thearea A and the area B are data obtained from the area A when theadditional area is created, the areas A and B may be restored to somedifferent values (including quantization error) during encoding/decodingprocess. Therefore, when restoring the division unit I, the portioncorresponding to the area A may be restored using the restored imagedata of the area A and the image data of the area B. For example, apartial area C of the division unit I may be replaced by an averages orweighted sum of the areas A and B. Since two or more pieces of data ofthe same area exist, a reconstructed image (area C. A of division unit Iis replaced with C) may be obtained by using data of two areas (theprocess is named Rec_Process in the drawing).

In addition, the partial areas C belonging to the division unit I may bereplaced using the areas A and B according to which division unit isclose. Specifically, since image data belonging to a specific range (forexample, M pixel intervals) to the left in the area C is adjacent to thedivision unit I, the image data may be restored by using (or copying)the data of the area A, and since the image data belonging to a specificrange (for example, N pixel intervals) to the right in the area C isadjacent to the division unit J, the image data may be restored by using(or copying) the data of the area B. This is represented by thefollowing equation 1.C(x,y)=A(x,y),(x,y)∈MB(x,y),(x,y)∈N

In addition, the partial area C belonging to the division unit I may bereplaced by weighting the image data of the areas A and B according towhich division unit is close. That is, the image data close to thedivision unit I in the area C may give high weight to the image data ofthe area A, and the image data close to the division unit J may givehigh weight to the image data in the area B. That is, the weight may beset based on a distance difference between the horizontal width of thearea C and the x coordinate of the pixel value to be corrected.

Equation 2 below may be derived as a formula for setting adaptiveweights for the areas A and B.C(x,y)=A(x,y)×w+B(x,y)×(1−w)w=f(x,y,k)

Referring to Equation 2, w means a weight given to the pixel coordinates(x, y) of the A area and the B area, In this case, in the sense of theweighted average of the areas A and B, the pixels of the area A aremultiplied by the weight w, and the pixels of the area B are multipliedby 1-w. However, in addition to the weighted average, the weights of thedifferent values may be given to the areas A and B, respectively.

As described above, when the additional area is used, the additionalarea B may be removed in a resizing process for the partition unit J andstored in the memory (DPB, which may be a decoded picture buffer) (Inthis example, it is assumed that an additional area positioning processis sizing. This process can be derived in some embodiments. <e.g.,through the process of checking for additional area flags, then checkingsize information, and then checking how to fill them up, etc.>. Assumethat while the process of getting smaller is performed in the sizingprocess, the process of getting smaller is performed in the resizingprocess).

In addition (In detail, immediately after image encoding/decoding), itmay be stored in the memory without performing the resizing process, andmay be removed by performing the resizing process in an output stage(assuming display phase in this example). This may be applied to all orsome division units belonging to the corresponding image.

The relevant setting information may be implicitly or explicitlyprocessed according to an encoding/decoding setting, and in the case ofimplicitly (in detail, depending on the characteristics, type, format,etc. of the image or based on other encoding/decoding settings (in thisexample, additional area related settings)), it may be determinedwithout generating a related syntax element, and the unit thereof mayinclude a video, a sequence, a picture, a subpicture, a slice, a tile,and the like.

On the other hand, the conventional coding method according to thedivision unit may include, 1) dividing a picture into one or more tiles(or generically referred to as division units), and generating divisioninformation, 2) performing encoding according to the divided tile unit,3) performing filtering based on information indicating whether to allowin-loop filters of tile boundaries, and 4) storing the filtered tile ina memory.

In addition, the decoding method according to the conventional divisionunit may include 1) dividing a picture into one or more tiles based ontile division information, 2) performing decoding on a divided tilebasis, 3) performing filtering based on information indicating whetherto allow in-loop filters on tile boundaries, and 4) storing the filteredtile in memory.

Herein, the third step of the encoding/decoding method is apost-processing step of encoding/decoding, and may be dependentencoding/decoding if filtering is performed, and independentencoding/decoding if filtering is not performed.

The encoding method of the division unit according to an embodiment ofthe present invention may include, 1) dividing a picture into one ormore tiles and generating division information, 2) setting an additionalarea for at least one divided tile unit, and filling the additional areausing adjacent tile units, 3) performing encoding on a tile unit havingan additional area, 4) removing the additional area on the tile unit,and performing filtering based on information indicating whether toallow an in-loop filter of a tile boundary, and 5) storing the filteredtile in memory.

In addition, a decoding method of a division unit of according to anembodiment of the present invention may include, 1) dividing a pictureinto one or more tiles based on tile division information, 2) setting anadditional area with respect to the divided tile unit, and filling theadditional area by using decoding information, preset information, oranother (adjacent) tile unit that is restored in advance, 3) performingdecoding on the tile unit in which the additional area is generated byusing the decoding information received from an encoding apparatus, 4)removing the additional area for the tile unit and performing filteringbased on information indicating whether to allow the in-loop filter fora tile boundary, and 5) storing the filtered tile in memory

In the encoding/decoding method of a division unit according to anembodiment of the present invention, the second step may be anencoding/decoding preprocessing process (if an additional area is set,dependent encoding/decoding, and otherwise, independentencoding/decoding). In addition, the fourth step may be anencoding/decoding post-processing process (if an additional area is set,dependent encoding/decoding, and otherwise, independentencoding/decoding). In this example, an additional area is used in theencoding/decoding process, and the size adjustment is performed to theinitial size of the tile before being stored in the memory.

First of all, when an encoder is described, a picture is divided intomultiple tiles. Depending on an explicit or implicit setting, additionalareas are put on the basis of a tile and relevant data is obtained fromadjacent areas. The encoding is performed on the basis of the updatedtile including existing tiles and additional areas. After the encodingis completed, the additional areas are removed and filtering isperformed according to an in-loop filtering application setting.

In this case, a filtering setting may be changed according to a methodof filling and removing an additional area. For example, in case ofsimple removal, the above in-loop filtering setting may be applied. Incase of removing using an overlapping area, filtering may be not appliedor other filtering setting may be applied. In other words, sinceoverlapping data may be utilized to reduce distortion of a tile boundaryarea much, regardless of whether the in-loop filter of a tile boundaryunit is applied, filtering may not be performed or a setting differentfrom the filtering setting (e.g., applying a filter with weak filterstrength to a tile boundary, etc.) in the tile may be applied dependingon whether the above filtering is applied. After the above process, itis saved in a memory.

Referring to a decoder, first, a picture is divided into multiple tilesaccording to tile division information transmitted from an encoder.Additional area related information is explicitly or implicitly checkedto parse encoding information of the updated tile transmitted from theencoder with the additional area. In addition, decoding is performed onthe basis of the updated tile. After the decoding is completed, theadditional area is removed and filtering is performed according to anin-loop filtering application setting similarly to the encoder. Variouscases of this have been described in an encoder, and thus a detaileddescription thereof will be omitted. After the above process, it issaved in a memory.

On the other hand, the case where an additional area for a division unitis used in an encoding/decoding process and stored in a memory withoutbeing deleted may be considered. For example, in the case of a360-degree image, an accuracy of prediction may be lowered in someprediction processes (e.g., inter prediction), depending on a surfacearrangement setting (e.g., when searching and compensating for motion,it is hard to find where the surface arrangement is not continuous).Therefore, in order to increase the accuracy of prediction, additionalareas may be stored in the memory and used in a prediction process. Ifutilized in inter prediction, the additional area (or a pictureincluding the additional area) may be used as a reference picture forinter prediction.

The encoding method according to the case where the additional area issaved may include, 1) dividing a picture into one or more tiles andgenerating division information, 2) setting an additional area for atleast one divided tile unit, and filling an additional area usingadjacent tile units, 3) performing encoding on a tile unit having theadditional area, 4) preserving additional areas for tile units (whereapplying in-loop filtering can be omitted), 5) storing the encoded tilein a memory.

The decoding method according to the case where the additional area issaved may include, 1) dividing a picture into one or more tiles based ontile division information, 2) setting an additional area with respect tothe divided tile unit, and filling the additional area by using decodinginformation, preset information, or another (adjacent) tile unit that isrestored in advance, 3) performing decoding on the tile unit in whichthe additional area is generated using decoding information receivedfrom the encoding apparatus, 4) preserving the additional area for tileunits (in-loop filtering can be omitted), 5) storing the decoded tile ina memory

Referring to an encoder according to the case of saving an additionalarea, a picture is divided into multiple tiles. Depending on an explicitor implicit setting, additional areas are put on the basis of a tile andrelevant data is obtained from preset areas. Since the preset arearefers to another area that is correlated according to a surfacearrangement setting in a 360-degree image, it may be an area adjacent toa current tile or an area not adjacent to the current tile. Encoding isperformed on the basis of the updated tile. Since the additional areawill be preserved after the encoding is completed, no filtering isperformed regardless of an in-loop filtering setting. The reason is thatboundaries of each updated tile do not share actual tile boundaries dueto the additional area. After the above process, it is saved in amemory.

Referring to a decoder according to the case of preserving an additionalarea, a picture checks tile division information transmitted from anencoder, and the picture is divided into multiple tiles accordingly. Theadditional area related information is checked and encoding informationof an updated tile transmitted from an encoder is parsed with theadditional area. Decoding is performed on the basis of the updated tile.After the decoding is completed, an in-loop filter is not applied in theadditional area and it is saved in a memory.

Hereinafter, an encoding/decoding method for a division unit accordingto an embodiment of the present invention will be described withreference to the drawings.

FIGS. 11 to 12 are exemplary diagrams for illustrating anencoding/decoding method for a division unit according to an embodimentof the present invention. Specifically, as an example in which anadditional area is generated for each division unit to performencoding/decoding, FIG. 11 illustrates an encoding method including anadditional area, and FIG. 12 illustrates a decoding method for removingthe additional area. In the case of a 360-degree image, a preprocessingprocess (stitching, projection, etc.) may be performed in the previousstep of FIG. 11, and a post processing process (rendering, etc.) may beperformed in the later step of FIG. 12.

First, referring to FIG. 11, in an encoder, when an input image isacquired (step A), an input image may be divided into two or moredivision units through a picture division unit (setting informationabout a division method may be generated at this time, indicated by stepB), an additional area for the division unit may be generated accordingto an encoding setting or whether the additional area is supported (stepC), and a bitstream may be generated by performing encoding on thedivision unit including the additional area (step D). In addition, aftergenerating the bitstream, it may be determined whether to resize (ordelete the additional area, step E) each division unit according to theencoding setting, the additional area may be included or the removedencoded data (picture according to D or E) may be stored in a memory(step E).

Referring to FIG. 12, in a decoder, an image to be decoded may bedivided into two or more division units by referring to division relatedconfiguration information obtained by parsing a received bitstream (stepB), a size of an additional area for each division unit may be setaccording to a decoding setting obtained in the received bitstream (stepC), and image data having an additional area may be obtained by decodingimage data included in the bitstream (step D). Next, a reconstructedpicture may be generated by deleting the additional area (step E), andthe reconstructed picture may be output to a display (step F). In thiscase, whether to delete the additional area may be determined accordingto a decoding setting, and the reconstructed picture or image data (dataaccording to D or E) may be stored in the memory. Meanwhile, in step F,a process of restoring a 360-degree image may be included throughsurface relocation of the reconstructed picture.

On the other hand, in-loop filtering (assuming a deblocking filter inthis example. Other in-loop filters may be applicable), may beadaptively performed at the partition unit boundary according to whetheradditional areas of FIG. 11 or 12 are removed. In addition, in-loopfiltering may be adaptively performed depending on whether to generatethe additional area is allowed.

When removing the additional area and storing in a memory (for tiles inthis example), the in-loop filter may or may not be explicitly applied,according to an in-loop filter application flag at a boundary of adivision unit such as loop_filter_across_enabled_flag (In detail, of theinitial state).

Alternatively, whether a flag for applying an in-loop filter is notsupported at a boundary of a division unit, and whether filtering isapplied and a filtering setting may be implicitly determined as in thefollowing example.

In addition, though there is continuity of the image between eachdivision unit, if an additional area is generated for each divisionunit, the continuity of the image with respect to a boundary between thedivision units in which the additional area is generated may be lost. Inthis case, when an in-loop filter is applied, unnecessary computationamount increases and encoding performance is deteriorated, so that thein-loop filter may not be applied implicitly.

In addition, there may be no image continuity between adjacent divisionunits in two-dimensional space according to an arrangement of surfacesin a 360-degree image. As such, when in-loop filtering is performed onboundaries between division units having no image continuity, imagequality degradation may occur. Therefore, the in-loop filtering may notbe implicitly performed on the boundaries between the division unitswithout the image continuity.

In addition, in the case of replacing some regions of a current divisionunit by weighting two regions as in the description of FIG. 10, aboundary of each division unit belongs to an inner boundary in anadditional area, and in-loop filtering may be applied. In addition, itmay not be necessary to perform in-loop filtering because encodingerrors may be reduced by a method such as a weight sum with a part of acurrent region belonging to another region. Therefore, in this case, thein-loop filter may not be implicitly performed.

In addition, whether to apply the in-loop filter may be determined basedon a flag indicating whether to apply the in-loop filter (In detail,additional to that boundary). When the flag is activated, filtering maybe applied according to the in-loop filter setting, condition, etc.applied inside a division unit, or filtering whose in-loop filtersetting, condition, etc. (in detail, additional unlike an in-loop filtersetting, condition, etc., if not the boundary of the division unit)applied to a boundary of the division unit are differently defined maybe applied.

In the above embodiment, though it is assumed that an additional area isremoved and stored in a memory, some of them may be processes that maybe performed at other output stages (In detail, it may belong to thein-loop filter unit and may also belong to the post filter unit, etc.).

The above example has been described under the assumption thatadditional areas are supported in each direction of each division unit,which may be applicable only to a part of the above contents when onlysome directions are supported according to a setting of the additionalareas. For example, a boundary in which the additional area is notsupported may be changed in various cases, such as following an existingconfiguration and the boundary in which the additional area issupported, following the case of the above example. That is, theapplication may be adaptively determined on all or part of the unitboundary according to the setting of the additional area.

The related setting information may be implicitly or explicitlyprocessed according to an encoding/decoding setting. If implicitly (indetail, depending on the characteristics, type, format, etc. of an imageor based on other encoding/decoding settings (in this example,additional area related settings)), the related setting information maybe determined without generating a related syntax element, and ifexplicitly, the related setting information may be adjusted bygenerating the related syntax element. The unit thereof may include avideo, a sequence, a picture, a subpicture, a slice, a tile, and thelike.

The following describes a method of determining whether a division unitand an additional area can be referred to. At this time, if it can bereferred to, it is dependent encoding/decoding, and if it cannot bereferred to, it can be independent encoding/decoding.

The additional area according to an embodiment of the present inventionmay be referred to or limited in an encoding/decoding process of acurrent image or another image. In detail, the additional area removedbefore being stored in a memory may be referred to or limited in theencoding/decoding process of the current image. In addition, theadditional area stored in the memory may be referred to or limited inthe encoding/decoding process of another image in time as well as thecurrent image.

In summary, a possibility of referencing and range of an additional areamay be determined according to an encoding/decoding setting. Accordingto some of the above settings, the additional area of the current imageis encoded/decoded and stored in the memory, which means that thereference may be included in the reference image of another image or thereference may be limited. This may be applied to all or some divisionunits belonging to the corresponding image. The case described in theexamples to be described later may be changed and applied to the presentexample.

Setting information on a possibility of referencing an additional areamay be implicitly or explicitly processed according to anencoding/decoding setting. If implicitly, it may be determined withoutgenerating relevant syntax elements (in detail, depending on thecharacteristics, type, format, etc. of the image or based on otherencoding/decoding settings (in this example, additional area relatedsettings)). If explicitly, setting on the possibility of referencing theadditional area may be adjusted by creating relevant syntax elements.The unit thereof may include a video, a sequence, a picture, asubpicture, a slice, a tile, and the like.

In general, some units (assuming a division unit acquired through apicture division unit in this example) in a current image may refer todata of the corresponding unit, and may not refer to data of otherunits. In addition, some units in the current image may refer to data ofall units existing in another image. The above description may be anexample of general properties in units obtained through the picturedivision unit, and additional properties may be defined.

In addition, a flag indicating whether it is possible to refer toanother division unit in the current image or whether a division unitbelonging to another image may be referred to may be defined.

For example, a division unit belonging to another image and belonging tothe same position as the current division unit may be referred to, andreference to a division unit having a different position from thecurrent division unit may be limited. For example, when multiplebitstreams encoded with the same image in different encoding settingsare transmitted, and a decoder selectively determines a bitstream fordecoding each region (division unit) of the image (in this example, itis assumed that decoding on the basis of a tile is performed), since apossibility of referencing between each division unit must be limitednot only in the same space but also in other spaces, encoding/decodingmay be performed so as to be referred to only in the same area ofanother image.

For example, a reference may be allowed or restricted according to theidentifier information on a division unit. For example, the identifierinformation allocated to the division unit may be referred to when theyare the same, and may not be referred to when they are not the same. Inthis case, the identifier information may refer to informationindicating that (dependent) encoding/decoding is performed in anenvironment that can be referred to each other.

The related setting information may be implicitly or explicitlyprocessed according to an encoding/decoding setting. If implicitly, therelated setting information may be determined without generating arelated syntax element, and if explicitly, the related settinginformation may be handled by generating the related syntax element. Theunit thereof may include a video, a sequence, a picture, a subpicture, aslice, a tile, and the like.

FIGS. 13a to 13g are exemplary diagrams for illustrating an area inwhich a specific division unit may be referred to. In FIGS. 13a to 13g ,areas shown by thick lines may refer to areas that can be referred to.

Referring to FIG. 13a , various reference arrows for performing interprediction may be identified. In this case, in the case of the C0 and C1blocks, the C0 and C1 blocks represent unidirectional inter prediction.The C0 block can obtain an RP0 reference block before the currentpicture and an RF0 reference block after the current picture. The C2block represents bidirectional inter prediction, and the RP1 and RF1reference blocks may be obtained from a previous picture of the currentpicture or a subsequent picture of the current picture. The figureillustrates an example of acquiring one reference block in the forwardand backward directions, but it may also be possible to acquire thereference block only in the forward direction or only in the backwarddirection. The C3 block represents the non-directional inter pictureprediction, and an RCO reference block can be obtained in the currentpicture. Although an example of obtaining one reference block is shownin the figure, it may also be possible when acquiring more than onereference block.

In the following example, a pixel value in inter prediction according toa division unit and a possibility of referencing prediction modeinformation will be mainly described, but other encoding/decodinginformation (e.g., intra prediction mode information, transform andquantization information, in-loop filter information, etc.) that can bereferred to spatially or temporally can be understood.

Referring to FIG. 13b , a current picture (Current (t)) is divided intotwo or more tiles, and blocks C0 of some tiles may obtain referenceblocks P0 and P1 by performing inter prediction in one direction. BlockC1 of some tiles may obtain reference blocks P3 and F0 by performinginter prediction in both directions. That is, it may be an example inwhich reference to a block belonging to another position of anotherimage is allowed without a limitation such as a position limitation or areference allowed only within the current picture.

Referring to FIG. 13c , a picture is divided into two or more tileunits, and some blocks C1 of some tiles may obtain reference blocks P2and P3 by performing inter prediction in one direction. Some blocks C0of some tiles may perform inter prediction in both directions to obtainreference blocks P0, P1, F0, and F1. Some blocks C3 of some tiles mayperform inter prediction in a non-directional manner to obtain thereference block FC0.

That is, in FIGS. 13b and 13c , it may be an example in which referenceto a block belonging to another position of another image is allowedwithout a limitation such as a position limitation or a referenceallowed only within the current picture.

Referring to FIG. 13d , a current picture is divided into two or moretile units, and the block C0 of some tiles may acquire the referenceblock P0 by performing inter prediction in all directions, but may notobtain the reference blocks P1, P2, and P3 belonging to some tiles. Theblock C4 of some tiles may acquire reference blocks F0 and F1 byperforming inter prediction in the backward direction, but may notobtain reference blocks F2 and F3. Some blocks C3 of some tiles mayacquire the reference block FC0 by performing inter prediction in anon-directional manner or may not obtain the reference block FC1.

That is, in FIG. 13d , reference may be possible or limited depending onwhether a picture (t−1, t, t+1 in this example) is divided, anencoding/decoding setting of a picture division unit. In detail,reference may be made only to a block belonging to a tile having thesame identifier information as the current tile.

Referring to FIG. 13e , the picture is divided into two or more tileunits, and some blocks C0 of some tiles may obtain reference blocks P0and F0 by performing inter prediction in both directions, or may notobtain reference blocks P1, P2, P3, F1, F2, and F3. That is, in FIG. 13e, reference may be made only to a tile at the same position as a tile towhich a current block belongs.

Referring to FIG. 13F, a picture is divided into two or more tile units,and some blocks C0 of some tiles may obtain reference blocks P1 and F2by performing inter prediction in both directions, or may not obtainreference blocks P0, P2, P3, F0, F1, and F3. In FIG. 13f , a bitstreamincludes information indicating a tile that can be referred to withrespect to a current division unit, and it may be an example ofidentifying a tile with reference to information.

Referring to FIG. 13g , a picture is divided into two or more tiles, andsome blocks C0 of some tiles may obtain reference blocks P0, P3, and P5by performing inter prediction in one direction, or may not obtainreference blocks P4. Some blocks C1 of some tiles may acquire referenceblocks P1, F0, and F2 by performing inter prediction in both directions,or may not obtain reference blocks P2 and F1.

FIG. 13g is an example that may be referred to or limited according towhether a picture (t−3, t−2, t−1, t, t+1, t+2, t+3 in this example) isdivided, an encoding/decoding setting (in this example, it is assumedthat information is determined by identifier information of a divisionunit, the identifier information of a picture unit, whether a partitionarea is the same area, whether a partition area is similar area,bitstream information of a division unit, etc.) of a division unit of apicture, and the like. Herein, a referenceable tile may have the same orsimilar position as a current tile in an image, have the same identifierinformation (in detail, in the picture unit or the division unit) as thecurrent tile, and may be the same as the bitstream from which thecurrent tile is obtained.

FIGS. 14a to 14e are exemplary diagrams for illustrating a possibilityof referencing for an additional area in a division unit according to anembodiment of the present invention. In FIGS. 14a to 14e , the areaindicated by the thick line means a referenceable area, and the areashown by the dotted line means an additional area for the division unit.

According to one embodiment of the present invention, referencepossibilities for some pictures (other pictures located before or aftertime) may be limited or allowed. In addition, a possibility ofreferencing of an entire extended division unit including an additionalarea may be limited or allowed. In addition, the possibility ofreferencing may be allowed or limited only for an initial division unitexcept for the additional area. In addition, the possibility ofreferencing of a boundary between the additional area and the initialdivision unit may be allowed or restricted.

Referring to FIG. 14a , some blocks C0 of some tiles may perform interprediction in one direction to obtain reference blocks P0 and P. Someblocks C2 of some tiles may perform inter prediction in both directionsto obtain reference blocks P2, P3, F0, and F1. Some blocks C1 of sometiles may perform inter prediction in a non-directional manner to obtainreference block FC0. Herein, the block C0 may acquire the referenceblocks P0, P1, P2, P3, and F0 in an initial tile area (a basic tileexcept an additional area) of some reference pictures t−1 and t+1, andthe block C2 may obtain not only the reference blocks P2 and P3 in theinitial tile area of the reference picture t−1, but also the referenceblock F1 in a tile area including the additional area in the referencepicture t+1. In this case, as can be seen in the reference block F1, areference block including a boundary between the additional area and theinitial tile area may be obtained.

Referring to FIG. 14b , some blocks C0, C1, and C3 of some tiles mayperform inter prediction in one direction to obtain reference blocks P0,P1, P2/F0, F2/F1, F3, and F4. Some blocks C2 of some tiles may performinter prediction in a non-directional manner to obtain reference blocksFC0, FC1, and FC2.

Some blocks C0, C1, and C3 may obtain reference blocks P0, F0, and F3 inan initial tile area of some reference pictures (t−1, t+1 in thisexample), the reference blocks P1, x, F4 at an updated tile regionboundary, and the reference blocks P2, F2, F1 outside the updated tilearea boundary.

Some blocks C2 may acquire the reference block FC1 in an initial tilearea of some reference picture (t in this example), the reference blockFC2 at an updated tile area boundary, the reference block FC0 outsidethe updated tile area boundary.

Herein, some blocks C0 may be blocks located in an initial tile area,some blocks C1 may be blocks located at an updated tile area boundary,and some blocks C3 may be blocks located outside an updated tileboundary.

Referring to FIG. 14c , a picture is divided into two or more tileunits, some pictures have additional areas in some tiles, some picturesdo not have additional areas in some tiles, and some pictures do nothave additional areas. Some blocks C0 and C1 of some tiles may obtainreference blocks P2, F1, F2, and F3 by performing inter prediction inone direction, or may not obtain reference blocks P0, P1, P3, and F0.Some blocks C2 of some tiles may obtain reference blocks FC1 and FC2 byperforming inter prediction in a non-directional manner or may notobtain reference blocks FC0.

Some blocks C2 may not obtain the reference block FC0 in an initial tilearea of some reference pictures (t in this example), and may obtain thereference block FC1 (FC0 and FC1 may be the same area in some additionalarea filling methods. FC0 may not be referenced in the initial tiledivision, but may be referenced when bringing the corresponding area tothe current tile through additional areas) in an updated tile area.

Some blocks C2 may obtain the reference block FC2 (basically, it is notpossible to refer to data of another tile of a current picture, but itis assumed that it can be referred to when a setting becomesreferenceable according to an identifier information in the aboveembodiment) in some tile regions of some reference pictures (t in thisexample).

Referring to FIG. 14d , a picture is divided into two or more tile unitsand has an additional area. Some blocks C0 of some tiles may obtainreference blocks P0, F0, F1, and F3 by performing inter prediction inboth directions, or may not obtain reference blocks P1, P2, P3, and F2.

In an initial tile area (tile 0) of some reference pictures t−1 of someblocks C0, the reference block P0 may be obtained, it is limited thatthe reference block P3 is obtained at an extended tile area boundary,and it is limited that the reference block P2 is obtained outside theboundary of the extended tile area (I.e., additional area).

In an initial tile area (tile 0) of some reference pictures t+1 of someblocks C0, the reference block F0 may be obtained, the reference blockF1 may be obtained at an extended tile region boundary, and thereference block F3 may be obtained outside the extended tile regionboundary.

Referring to FIG. 14e , a picture is divided into two or more tiles andhas an additional area having at least one size and shape. The block C0of some tiles may acquire the reference blocks P0, P3, P5, and F0 byperforming inter prediction in one direction, but may not obtain thereference block P2 located at a boundary between the additional area andthe base tile. The block C1 of some tiles may obtain the referenceblocks P1, F2, and F3 by performing inter prediction in both directions,but may not obtain the reference blocks P4, F1, and F5.

As in the above example, the pixel value may be a reference object, andreference restriction on other encoding/decoding information may bepossible.

For example, when a prediction unit finds an intra prediction modecandidate group in intra prediction in a spatially adjacent block,whether a division unit to which a current block belongs may refer to adivision unit to which an adjacent block belongs may be checkedaccording to the method as illustrated in FIGS. 13a to 14 e.

For example, when a prediction unit finds a motion information candidategroup in inter prediction in a temporally and spatially adjacent block,whether a division unit to which a current block belongs may refer to adivision unit to which a block adjacent to the current picture isspatially or temporally adjacent to the current picture may be checkedaccording to the method of FIGS. 13a to 14 e.

For example, when in-loop filter related setting information is found inan adjacent block, whether a division unit to which a current blockbelongs may refer to a division unit to which an adjacent block belongsmay be checked according to the method as illustrated in FIGS. 13a to 14e.

FIG. 15 is an exemplary diagram of a block belonging to a division unitof a different image from a block belonging to a division unit of acurrent image.

Referring to FIG. 15, blocks in the left, top left, bottom left, top,and top right of a current block may be spatially adjacent referencecandidate blocks. In addition, the left, top left, bottom left, top, topright, right, bottom right, bottom, and center blocks of a block(Collocated block) at the same or corresponding position as the currentblock in an image (Different picture) temporally adjacent to the currentimage (Current picture) may be temporal reference candidate blocks. InFIG. 15, the thick outline is a boundary line which shows a divisionunit.

When the current block is M, all spatially adjacent blocks G, H, I, L,and Q may be referable.

When a current block is G, some of the spatially adjacent blocks A, B,C, F, and K may be referable, and the remaining blocks may be limited inreference. Whether reference is possible may be determined according toa reference related setting of a division unit (UC, ULC, LC) to which aspatially adjacent block belongs and a division unit to which a currentblock belongs.

When a current block is S, some of the blocks (s, r, m, w, n, x, t, o,y) around the same position as the current block in a temporallyadjacent image may be referable, and the remaining blocks may be limitedin reference. Whether the reference is possible may be determinedaccording to a reference-related setting of a division unit (RD, DRD,DD) to which a block around the same position as the position of thecurrent block in the temporally adjacent image and a unit to which thecurrent block belongs.

If there is a candidate whose reference is restricted according to aposition of a current block, a candidate of the next order among acandidate group configuration may be filled, or another candidateadjacent to the candidate whose reference is restricted may be replaced.

For example, in intra prediction, when a current block is G, the topleft block is restricted in reference, and a MPM candidate groupconfiguration follows the order of P-D-A-E-U, since A may not bereferenced, validation may be made in the order of the remaining E-U toform a candidate group, or B or F spatially adjacent to A may besubstituted for A.

Alternatively, in the inter prediction, when the current block is S, thetemporally adjacent bottom left block is limited in reference, and atemporal candidate configuration of the skip mode candidate group is y,since y is impossible to refer to, validation may be made in the orderof spatially adjacent candidates or mixed candidates of spatialcandidates and temporal candidates to form the candidate group, or y maybe replaced by t, x, and s adjacent to y.

FIG. 16 is a hardware block diagram of an image encoding/decodingapparatus according to an embodiment of the present invention.

Referring to FIG. 16, the image encoding/decoding apparatus 200according to an embodiment of the present invention may include at leastone processor 210 and the memory 220 storing instructions instructing atleast one processor 210 to perform at least one step.

Herein, the at least one processor 210 may mean a central processingunit (CPU), a graphics processing unit (GPU), or a dedicated processoron which methods according to embodiments of the present invention areperformed. Each of the memory 120 and the storage device 260 may beconfigured as at least one of a volatile storage medium and anonvolatile storage medium. For example, the memory 220 may beconfigured as at least one of a read only memory (ROM) and arandom-access memory (RAM).

In addition, the image encoding/decoding apparatus 200 may include thetransceiver 230 that performs communication through a wireless network.In addition, the image encoding/decoding device 200 may further includethe input interface device 240, the output interface device 250, thestorage device 260, and the like. Each component included in the imageencoding/decoding apparatus 200 may be connected by the bus 270 tocommunicate with each other.

Herein, at least one step may include dividing an encoded image includedin a bitstream into at least one division unit by referring to a syntaxelement obtained from a received bitstream, setting an additional areafor the at least one division unit, and decoding the encoded image basedon the division unit in which the additional area is set.

Herein, the decoding of the encoded image may include, according toinformation indicating a possibility of referencing included in thebitstream, determining a reference block for the current block to bedecoded in the encoded image.

Herein, the reference block may be a block belonging to a positionoverlapping the additional area set in the division unit to which thereference block belongs.

FIG. 17 is an exemplary diagram for illustrating an intra predictionmode according to an embodiment of the present invention.

Referring to FIG. 17, 35 prediction modes may be identified, and 35prediction modes may be classified into 33 directional modes and twonon-directional modes (DC, planar). In this case, the directional modemay be identified by inclination (for example, dy/dx) or angleinformation. The above example may mean a prediction mode candidategroup for a luminance component or a chrominance component.Alternatively, some prediction modes (e.g., DC, planar, vertical,horizontal, diagonal modes, etc.) may be supported for the colordifference component. In addition, when the prediction mode of theluminance component is determined, the corresponding mode may beincluded as the prediction mode of the chrominance component, or a modederived from the corresponding mode may be included as the predictionmode.

In addition, a reconstruction block of another color space in whichencoding/decoding is completed may be used for prediction of the currentblock by using the correlation between color spaces, and may include aprediction mode supporting the same. For example, in the case of achrominance component, a reconstructed block of luminance componentscorresponding to the current block may be generated as a predictionblock of the current block.

The prediction mode candidate group may be adaptively determinedaccording to an encoding/decoding setting. The number of candidates inthe group may be increased for the purpose of improving the accuracy ofprediction, and the number of candidates in the group may be reduced forthe purpose of reducing the amount of bits according to a predictionmode.

For example, one of candidate groups such as A candidate group (67.65directional modes and two non-directional modes), B candidate group(35.33 directional modes and two non-directional modes), and C candidategroup (19.17 directional modes and two non-directional modes) may beused. Unless otherwise described in the present invention, it is assumedthat intra prediction is performed with one preset prediction modecandidate group (A candidate group).

FIG. 18 is a first exemplary diagram of a reference pixel configurationused for intra prediction according to an embodiment of the presentinvention.

An intra prediction method in image encoding according to an embodimentof the present invention may include constructing a reference pixel,generating a prediction block using at least one prediction mode withreference to the constructed reference pixel, determining an optimalprediction mode, and encoding the determined prediction mode. Inaddition, the apparatus for encoding an image may be configured toinclude a reference pixel constructing unit for implementing a referencepixel constructing step, a prediction block generation unit forimplementing a prediction block generating step, a prediction modedeterminer for implementing a prediction mode determining step, and aprediction mode encoder for implementing a prediction mode encodingstep. Some of the processes described above may be omitted or otherprocesses may be added, and may be changed in an order other than theorder described above.

Meanwhile, in an intra prediction method according to an embodiment ofthe present invention, a reference pixel may be configured and aprediction block for the current block may be generated according to aprediction mode obtained through syntax information received by an imageencoding apparatus.

A size and shape (M×N) of a current block on which intra prediction isperformed may be obtained from a block division unit and may have a sizeof 4×4 to 256×256. Intra prediction may be generally performed on thebasis of a prediction block, but may be performed on the basis of acoding block (or a coding unit), a transform block (or a transformationunit), or the like, according to a setting of the block division unit.After checking block information, a reference pixel configuration unitmay configure a reference pixel used for prediction of the currentblock. In this case, the reference pixel may be managed through atemporary memory (for example, an array. primary or secondary array,etc.), generated and removed for each intra prediction process of theblock, and a size of the temporary memory may be determined according tothe reference pixel configuration.

A reference pixel may be a pixel belonging to an adjacent block (may bereferred to as a reference block) located at the left, top, top left,top right, and bottom left of a current block, but a block candidategroup having another configuration may be used for prediction of thecurrent block. Herein, adjacent blocks located at the left, top, topleft, top right, and bottom left may be selected blocks according to thecase where encoding/decoding is performed according to a raster or Zscan. It may be a reference pixel. If the scanning order is different, apixel belonging to the adjacent block at different position may also bea reference pixel.

In addition, a reference block may be a block corresponding to thecurrent block in a color space different from the color space to whichthe current block belongs. Herein, when Y/Cb/Cr format is taken as anexample, the color space may mean one of Y, Cb, and Cr. In addition, theblock corresponding to the current block may mean a block having thesame position coordinates as the current block or having a positioncoordinate corresponding to the current block according to a compositionratio of color components.

In addition, for convenience of description, a reference block accordingto a preset position (left, top, top left, top right, bottom left) isdescribed on the premise that one block is configured, but it may becomposed of multiple sub blocks according to block division.

In summary, an adjacent region of a current block may be a position of areference pixel for intra prediction of the current block, and an areacorresponding to the current block of another color space may be furtherconsidered as a position of the reference pixel according to aprediction mode In addition to the above example, a position of areference pixel defined according to a prediction mode, a predictionmethod, or the like may be determined. For example, when generating aprediction block through a method such as block matching, a regionincluded in a search range in a region where encoding/decoding iscompleted or a region where encoding/decoding is completed before acurrent block of a current image may be considered as a position of thereference pixel.

Referring to FIG. 18, a reference pixel used for intra prediction of acurrent block (the size of M×N) may be compose of left, right, top left,top right, and bottom left adjacent pixels (Ref_L, Ref_T, Ref_TL,Ref_TR, Ref_BL) of the current block. In this case, what is representedin the form of P (x, y) in FIG. 18 may mean pixel coordinates.

Meanwhile, pixels adjacent to the current block may be classified intoat least one reference pixel layer. The pixels closest to the currentblock may be classified into ref_0 {pixels having pixel value differenceof 1 with a boundary pixel of the current block. p(−1, −1)˜p(2M−1, −1),p(−1, 0) p(−1, 2N−1)}, the next adjacent pixel ref_1 {pixels havingpixel value difference of 2 with a boundary pixel of the current block.p (−2, −2) to p(2M, −2), p (−2, −1) to p(−2, 2N)}, and the next adjacentpixel ref_2 {pixels having pixel value difference of 3 with a boundarypixel of the current block. p (−3, −3) to p(2M+1, −3), p (−3, −2) top(−3, 2N+1)}. That is, reference pixels may be classified into multiplereference pixel layers according to pixel distances adjacent to boundarypixels of the current block.

In addition, a reference pixel layer may be set differently for eachadjacent neighboring block. For example, when using a block neighboringthe top of a current block as a reference block, a reference pixelaccording to layer ref_0 is used, and when a block adjacent to the topright of the current block is used as a reference block, a referencepixel according to layer ref_1 may be used.

Herein, in general, a reference pixel set referred to when performingintra prediction may belong to neighboring blocks adjacent to a currentblock in the bottom left, left, top left, top, and top right, and may bea pixel belonging to the layer ref_0 (a pixel closest to a boundarypixel). Unless otherwise described below, it is assumed that thesepixels. However, only pixels belonging to some of the aforementionedneighboring blocks may be used as the reference pixel set, or pixelsbelonging to two or more layers may be used as the reference pixel set.Herein, the reference pixel set or the layer may be implicitlydetermined (preset in an encoding/decoding apparatus) or may beexplicitly determined (determined by information received from theencoding apparatus).

Herein, although the supported reference pixel layer is described on thepremise of a maximum of three, it may have more values. The number ofreference pixel sets (or may be referred to as a reference pixelcandidate group) according to the number of reference pixel layers and aposition of referenceable neighboring blocks may be set differentlyaccording to a size, shape, prediction mode, image type <I/P/B. in thiscase, the image is a picture, slice, tile, etc.>, color component, etc.of the block. The related information may be included in a unit such asa sequence, a picture, a slice, a tile, and the like.

Although the present invention is described on a premise of allocating alow index (incremented by 1) from the reference pixel layer closest to acurrent block, the present invention is not limited thereto. Inaddition, information related to a reference pixel configuration to bedescribed later may be generated under the above index setting (such asbinarization, which assigns short bits to a small index when selectingone of multiple reference pixel sets).

In addition, when there are two or more supported reference pixellayers, a weighted average, etc. may be applied using each referencepixel included in the two or more reference pixel layers.

For example, a prediction block may be generated by using a referencepixel obtained by a weighted sum of pixels located in layers ref_0 andref_1 of FIG. 18. In this case, the pixel to which the weighted sum isapplied in each reference pixel layer may be a decimal unit pixel aswell as an integer unit pixel according to a prediction mode (e.g.,prediction mode directionality, etc.). In addition, one prediction blockmay be obtained by weighting a prediction block obtained by using areference pixel according to the first reference pixel layer and aprediction block obtained by using the reference pixel according to thesecond reference pixel layer. In this case, the weight may have a higherweight as a prediction block according to a reference pixel layeradjacent to the current block.

Assuming that information is explicitly generated in relation to areference pixel configuration, indication information (In this example,adaptive_intra_ref_sample_enabled_flag) allowing an adaptive referencepixel configuration may occur on the basis of a video, a sequence, apicture, a slice, a tile, and the like.

When the indication information means that an adaptive reference pixelconfiguration is allowed (in this example,adaptive_intra_ref_sample_enabled_flag=1), information (in this example,adaptive_intra_ref_sample_flag) of an adaptive reference pixelconfiguration may occur on the basis of a picture, a slice, a tile, ablock, and the like.

When the configuration information means an adaptive reference pixelconfiguration (in this example, adaptive_intra_ref_sample_flag=1),information of a reference pixel configuration related (e.g., selectioninformation about a reference pixel layer and a reference pixel set,etc. in this example intra_ref_idx) may be generated on the basis of apicture, a slice, a tile, a block, and the like.

In this case, when an adaptive reference pixel configuration is notallowed or a reference pixel configuration is not adaptive, thereference pixel may be configured according to a predetermined setting.In general, to configure the most adjacent pixel of a neighboring blockas the reference pixel is an example of that, but the present inventionis not limited thereto, and a variety of cases (e.g., when ref_0 andref_1 are selected as reference pixel layers and prediction pixel valuesare generated through a method such as weighted sum through ref_0 andref_1, i.e., if implicitly) may be possible.

In addition, information related to a reference pixel configuration(e.g., selection information about a reference pixel layer or set, etc.)may be configured (e.g. ref_1, ref_2, ref_3, etc.) except for presetinformation (e.g., if the reference pixel layer is preset to ref_0), butis not limited thereto.

Some examples of the reference pixel configuration have been describedthrough the above example, which may be combined with variousencoding/decoding information and the like to determine an intraprediction setting. In this case, encoding/decoding information maycorrespond to an image type, a color component, a size, a shape of acurrent block, a prediction mode {type of prediction mode (directional,non-directional), direction of prediction mode (vertical, horizontal,diagonal 1, diagonal 2, etc.)}, and the like. An intra predictionsetting (in this example, a setting of reference pixel configuration)may be determined according to encoding/decoding information of aneighboring block and a combination of encoding/decoding information ofa current block and a neighboring block

FIGS. 19a to 19c are second exemplary diagrams of reference pixelconfigurations according to an exemplary embodiment of the presentinvention.

Referring to FIG. 19a , a case in which a reference pixel is configuredusing only the reference pixel layer ref_0 of FIG. 18 may be confirmed.Subsequent intra prediction (some intra prediction may be adaptivelyperformed depending on the reference pixel configuration such asreference pixel generation, reference pixel filtering, reference pixelinterpolation, prediction block generation, post-processing filtering)may be performed after configuring the reference pixel by using pixelsbelonging to a neighboring block (e.g., bottom left, left, top left,top, top right) for the reference pixel layer ref_0. In this example,when one preset reference pixel layer is used, the example illustratesthat setting information about the reference pixel layer is notgenerated, and intra prediction is performed using a non-directionalprediction mode.

Referring to FIG. 19b , it may be confirmed that a reference pixel isconfigured using both supported reference pixel layers. That is, afterconfiguring the reference pixel using pixels belonging to the layerref_0 and the layer ref_1 (or using a weighted average of the pixelsbelonging to two layers), intra prediction may be performed. In thisexample, when multiple preset reference pixel layers are used, theexample illustrates that setting information about the reference pixellayer is not generated, and intra prediction is performed using somedirectional prediction modes (from top right to bottom left or viceversa in the drawing).

Referring to FIG. 19c , it may be confirmed that a reference pixel isconfigured using only one reference pixel layer among three supportedreference pixel layers. This example illustrates multiple referencepixel layer candidates exist, setting information on the reference pixellayer used therein is generated, and intra prediction is performed usingsome directional prediction modes (from top left to bottom right in thedrawing).

FIG. 20 is a third exemplary diagram of a reference pixel configurationaccording to an embodiment of the present invention.

In FIG. 20, the symbol a is a block having a size of 64×64 or more, thesymbol b is a block having a size of 16×16 or more and less than 64×64,and the symbol c is a block having a size of less than 16×16.

If a block according to the symbol a is a current block to perform intraprediction, the intra prediction may be performed by using the onereference pixel layer ref_0 that is the nearest.

In addition, if a block according to the symbol b is a current block toperform intra prediction, the intra prediction may be performed by usingthe two supportable pixel layers ref_0 and ref_1.

In addition, if a block according to the symbol c is a current block toperform intra prediction, the intra prediction may be performed by usingthe three supportable pixel layers ref_0, ref_1, and ref_2.

Referring to the description according to the symbols a to c, the numberof supportable reference pixel layers may be determined differentlyaccording to a size of a current block on which intra prediction is tobe performed. In FIG. 20, the larger a size of a current block is, thehigher a probability that a size of a neighboring block is smaller.Since this may be a result of division due to characteristics of adifferent image, for the purpose of preventing prediction from a pixelhaving a large pixel value distance from the current block, it isassumed that as a size of a block increases, the number of supportedreference pixel layers decreases, but other variations including theopposite are possible

FIG. 21 is a fourth exemplary diagram of a reference pixel configurationaccording to an embodiment of the present invention.

Referring to FIG. 21, it may be confirmed that a current block forperforming intra prediction is in a rectangular shape. If the currentblock is rectangular and the width and length are asymmetric, it ispossible to set the support number of a reference pixel layer adjacentto the long horizontal boundary surface in the current block to belarge, and it is possible to set the support number of the adjacentreference pixel layer adjacent to the short vertical boundary surface inthe current block to be smaller. In the drawing, it may be confirmedthat the reference pixel layer adjacent to the horizontal boundary ofthe current block is set to two, and the reference pixel layer adjacentto the vertical boundary of the current block is set to one. This isbecause pixels adjacent to the short vertical boundary in the currentblock are often far from pixels included in the current block (becausethe length is long), so that an accuracy of prediction may be reduced.Accordingly, although the support number of the reference pixel layeradjacent to the short vertical boundary surface is set to be smaller,the reverse may also be possible.

In addition, a reference pixel layer to be used for prediction may bedifferently determined according to a type of an intra prediction modeor a position of a neighboring block adjacent to a current block. Forexample, a directional mode using a pixel belonging to a block adjacentto the top, top right of the current block as a reference pixel may usetwo or more reference pixel layers, a directional mode using a pixelbelonging to a block adjacent to the left, bottom left of the currentblock as a reference pixel may use only one nearest reference pixellayer.

On the other hand, if prediction blocks generated through each referencepixel layer in multiple reference pixel layers are identical or similarto each other, generating setting information of the reference pixellayer may be a result of generating unnecessary data additionally.

For example, when distribution characteristics of pixels constitutingeach reference pixel layer are similar or identical to each other, sincesimilar or identical prediction blocks may be generated using anyreference pixel layer, it is not necessary to generate data forselecting the reference pixel layer. In this case, the distributioncharacteristics of the pixels constituting the reference pixel layer maybe determined by comparing an average or dispersion value of the pixelswith a preset threshold value.

That is, when reference pixel layers are identical or similar to eachother based on the finally determined intra picture prediction mode, thereference pixel layer may be selected by a preset method (e.g.,selecting the nearest reference pixel layer).

In this case, a decoder may receive intra prediction information (orintra prediction mode information) from an encoding apparatus, anddetermine whether to receive information for selecting a reference pixellayer based on the received information.

Although a reference pixel is configured by using multiple referencepixel layers through various examples, various examples of modificationsare possible and may be combined with other additional configurations.

The reference pixel unit of intra prediction may include a referencepixel generation unit, a reference pixel interpolation unit, a referencepixel filter unit, and the like, and may include all or part of theabove components. Herein, a block including pixels that may becomereference pixels may be referred to as a reference candidate block. Inaddition, the reference candidate block may generally be a neighboringblock adjacent to a current block.

According to an availability of reference pixel to be set for thereference candidate block in a reference pixel configuration unit, itmay be determined whether a pixel belonging to the reference candidateblock is used as the reference pixel.

The reference pixel availability may be determined to be unavailablewhen at least one of the following conditions is satisfied. For example,at least one of the case that the reference candidate block is locatedoutside the picture boundary, the case that it does not belong to thesame division unit as the current block (for example, slice, tile,etc.), the case that encoding/decoding is not completed, or the casethat any of the cases where the use is restricted according toencoding/decoding setting is satisfied, it may be determined that thepixels belonging to the reference candidate block cannot be referred to.In this case, when all of the above conditions are not satisfied, it maybe determined that it can be referred to.

In addition, a use of a reference pixel may be restricted by anencoding/decoding setting. For example, when a flag for restricting areference to a reference candidate block (e.g.,constrained_intra_pred_flag) is activated, a pixel belonging to thereference candidate block may not be used as a reference pixel. The flagmay be applied when the reference candidate block is a blockreconstructed with reference to an image different in time from thecurrent picture in order to perform encoding/decoding that is robust toerrors due to various external factors including a communicationenvironment.

Herein, when the flag for restricting the reference is deactivated(e.g., constrained_intra_pred_flag=0 in an I picture type or a P or Bpicture type), all the pixels of the reference candidate block may beused as the reference pixels. In addition, depending on whether thereference candidate block is encoded with intra prediction or interprediction, when the flag limiting the reference is activated (e.g.,constrained_intra_pred_flag=1 in the P or B picture type), it may bedetermined whether reference is possible. That is, if the referencecandidate block is encoded by intra prediction, the reference candidateblock may be referred to regardless of whether the flag is activated. Ifthe reference candidate block is encoded by inter prediction, whetherthe reference candidate block can be referred to may be determinedaccording to whether the flag is activated.

In addition, a reconstruction block having a position corresponding tothe current block in another color space may be a reference candidateblock. In this case, whether reference is possible may be determinedaccording to an encoding mode of the reference candidate block. Forexample, if the current block belongs to some chrominance components Cband Cr, whether reference is possible may be determined according to anencoding mode of a block having a position corresponding to the currentblock in a luminance component Y and having been encoded/decoded(=reference candidate block). This may be an example corresponding tothe case where the encoding mode is independently determined accordingto the color space.

A flag for limiting a reference may be a setting applied to some imagetypes (e.g., P or B slice/tile type, etc.).

Through an availability of reference pixels, reference candidate blocksmay be classified as being available, partially available, or notavailable at all. In all cases except when all are available, referencepixels at unavailable candidate block positions may be filled orgenerated.

When the reference candidate block is available, a pixel (or a pixeladjacent to the current block) at a predetermined position of thecorresponding block may be stored in a reference pixel memory of thecurrent block. In this case, pixel data of a position of thecorresponding block may be copied as it is or stored in the referencepixel memory through a process such as reference pixel filtering.

When the reference candidate block is unavailable, a pixel acquiredthrough the reference pixel generation process may be included in thereference pixel memory of the current block.

In summary, a reference pixel may be configured when the reference pixelcandidate block is in an available state, and the reference pixel may begenerated when the reference pixel candidate block is in an unavailablestate.

A method of filling a reference pixel at a preset position in anunavailable reference candidate block is as follows. First, a referencepixel may be generated using an arbitrary pixel value. Herein, thearbitrary pixel value may be a specific pixel value belonging a thepixel value range, may be a minimum value, maximum value, or medianvalue of a pixel value used in a pixel value adjustment process based ona bit depth or a pixel value adjustment process based on pixel valuerange information of an image, and may be a value derived from thevalues. Herein, generating a reference pixel with the arbitrary pixelvalue may be applied when all reference candidate blocks areunavailable.

Next, a reference pixel may be generated using pixels belonging toblocks adjacent to an unavailable reference candidate block. In detail,pixels belonging to an adjacent block may be filled by extrapolation,interpolation, or copying at a predetermined position in the unavailablereference candidate block. In this case, a direction of performingcopying or extrapolation may be clockwise or counterclockwise, and maybe determined according to an encoding/decoding setting. For example, adirection of generating the reference pixel in a block may follow onepreset direction or may follow an adaptively determined directiondepending on a location of the unavailable block.

FIGS. 22a to 22b are exemplary diagrams of a method of filling areference pixel at a preset position in an unavailable referencecandidate block.

Referring to FIG. 22a , a method of filling pixels belonging to anunavailable reference candidate block among reference pixels having onereference pixel layer may be confirmed. In FIG. 22a , when a neighboringblock adjacent to the top right of a current block is the unavailablereference candidate block, a reference pixel (indicated by <1>)belonging to the neighboring block adjacent to the top right of thecurrent block may be generated by extrapolating or linearlyextrapolating a reference pixel belonging to a neighboring blockadjacent to the top of the current block in a clockwise direction.

In addition, in FIG. 22a , when a neighboring block adjacent to the leftof the current block is an unavailable reference candidate block,reference pixels belonging to the neighboring block adjacent to the left(denoted by <2>) may be generated by extrapolating or linearlyextrapolating a reference pixel belonging to a neighboring blockadjacent to the top left of the current block (corresponding to anavailable block) in a counterclockwise direction. In this case, whenextrapolation or linear extrapolation is performed in a clockwisedirection, reference pixels belonging to a neighboring block adjacent tothe bottom left of the current block may be used.

In addition, in FIG. 22a , a part of the reference pixels (indicated by<3>) belonging to a neighboring block adjacent to the top of the currentblock may be generated by interpolating or linearly interpolating thereference pixels usable on both sides. That is, a case in which some butnot all of the reference pixels belonging to the neighboring blocks areunavailable may be set, and in this case, unavailable reference pixelsmay be filled using adjacent pixels of the unavailable reference pixels.

Referring to FIG. 22b , a method of filling a reference pixel that isunavailable when some reference pixels are not available among referencepixels composed of multiple reference pixel layers may be confirmed.Referring to FIG. 22b , when a neighboring block adjacent to the topright of the current block is an unavailable reference candidate block,pixels belonging to three reference pixel layers belonging to thecorresponding neighboring block (denoted by <1>) may be generated in aclockwise direction using pixels belonging to a neighboring block(corresponding to an available block) adjacent to the top of the currentblock.

In addition, in FIG. 22b , if a neighboring block adjacent to the leftof the current block is an unavailable reference candidate block, and aneighboring block adjacent to the top left and bottom left of thecurrent block is an available reference candidate block, referencepixels of the available reference candidate block may be filled in aclockwise, counterclockwise or bidirectional direction to generatereference pixels of the unavailable reference candidate block.

In this case, an unavailable reference pixel of each reference pixellayer may be generated using pixels of the same reference pixel layer,but a use of pixels of non-identical reference pixel layers is notexcluded. For example, in FIG. 22b , reference pixels (denoted by <3>)according to three reference pixel layers belonging to a neighboringblock adjacent to the top of the current block are assumed to beunavailable reference pixels. In this case, pixels belonging to thereference pixel layer ref_0 that is farthest from the reference pixellayer ref_0 that is closest to the current block may be generated byusing reference pixels that belong to the same reference pixel layer andare available. In addition, pixels belonging to the reference pixellayer ref_1 separated by one pixel from the current block may begenerated by using not only pixels belonging to the same reference pixellayer ref_1 but also pixels belonging to other reference pixel layersref_0 and ref_2. In this case, reference pixels available on both sidesmay be filled with reference pixels which are not available by using amethod such as quadratic linear interpolation.

The above example shows an example of generating a reference pixel whensome reference candidate blocks are unavailable when multiple referencepixel layers are configured as reference pixels. Alternatively, asetting (e.g., when at least one reference candidate block isunavailable or all reference candidate blocks are unavailable) that doesnot allow an adaptive reference pixel configuration (in this example,adaptive_intra_ref_sample_flag=0) may be possible according to anencoding/decoding setting. In other words, the reference pixel may beconfigured according to a predetermined setting without any additionallyoccurring information.

The reference pixel interpolation unit may generate reference pixels ina decimal unit through linear interpolation of a reference pixel. In thepresent invention, it is assumed and described as some processes of thereference pixel configuration unit, but it may be included in theprediction block generation unit and may be understood as a processperformed before generating a prediction block.

In addition, although it is assumed to be a separate process from thereference pixel filter unit to be described later, it may be integratedinto one process. This may be configured to prepare for the case wheredistortion is generated in a reference pixel by increasing the number offiltering applied to the reference pixel when a plurality of filteringis applied through the reference pixel interpolation unit and thereference pixel filter unit.

The reference pixel interpolation process is not performed in someprediction modes (e.g., horizontal, vertical, some diagonal modes <modeshaving 45 degree angles such as Diagonal down right, Diagonal down left,Diagonal up right>, non-directional mode, color mode, color copy mode,etc., that is, modes that does not require interpolation in a decimalunit when generating a prediction block) and may be performed in otherprediction modes (that is, modes that require interpolation in a decimalunit when generating a prediction block).

Interpolation precision (e.g., a pixel unit such as 1, ½, ¼, ⅛, 1/16,1/32, 1/64, etc.) may be determined according to a prediction mode (or aprediction mode directionality). For example, an interpolation processis not necessary in a prediction mode having a 45 degree angle, and aninterpolation in a ½ pixel unit is required in a prediction mode havinga 22.5 degree angle or 67.5 degree angle. As described above, at leastone interpolation precision and a maximum interpolation precision may bedetermined according to the prediction mode.

For reference pixel interpolation, only one preset interpolation filter(e.g., 2-tap linear interpolation filter) may be used, or a filterselected according to an encoding/decoder setting among multipleinterpolation filter candidate groups (e.g., 4-tap cubic filter, 4-tapGaussian filter, 6-tap winner filter, 8-tap Kalman filter, etc.) may beused. In this case, the interpolation filter may be classified into adifference in the number of filter taps (that is, the number of pixelsto which filtering is applied) and a filter coefficient.

Interpolation may be performed step by step in order of low precision tohigh precision (e.g., ½→¼→⅛), or may be performed in batches. In theformer case, interpolation may be performed based on pixels in integerunits and pixels in decimal units (pixels previously interpolated with alower precision than an interpolation target pixel at present). In thelatter case, interpolation may be performed based on pixels of aninteger unit.

When using one of multiple filter candidate groups, filter selectioninformation may be explicitly generated or implicitly determined, andmay be determined according to an encoding/decoding setting (e.g.interpolation precision, block size, shape, prediction mode, etc.). Inthis case, a unit explicitly generated may be a video, a sequence, apicture, a slice, a tile, a block, and the like.

For example, when an interpolation precision (½, ¼) of ¼ or more isapplied, an 8-tap Kalman filter may be applied to a reference pixel ininteger units. When the interpolation precision (⅛, 1/16) of less than ¼and more than 1/16, a 4-tap Gaussian filter may be applied to thereference pixel in integer units and an interpolated reference pixel onthe basis of ¼ or more, When the interpolation precision ( 1/32, 1/64)is less than 1/16, a 2-tap linear filter may be applied to the referencepixel in integer units and the interpolated reference pixel on the basisof 1/16 or more.

Alternatively, for blocks of more than 64×64, an 8-tap Kalman filter maybe applied, for blocks of less than 64×64 and 16×16 or more, a 6-tapwinner filter may be applied, and for blocks of less than 16×16, a 4-tapGaussian filter may be applied.

Alternatively, a 4-tap cubic filter may be applied to a prediction modein which an angle difference is less than 22.5 degrees based on thevertical or horizontal mode, and a 4-tap Gaussian filter may be appliedto a prediction mode in which an angle difference is 22.5 degrees ormore.

Alternatively, multiple filter candidate groups may be configured with a4-tap cubic filter, a 6-tap winner filter, and an 8-tap Kalman filter insome encoding/decoding settings, or may be configured with a 2-taplinear filter or a 6-tap winner filter in some encoding/decodingsettings.

FIGS. 23a to 23c are exemplary diagrams of a method of performinginterpolation based on a fractional pixel unit for reference pixelsconfigured according to an embodiment of the present invention.

Referring to FIG. 23a , when one reference pixel layer ref_i issupported as reference pixels, a method of interpolating pixels in adecimal unit may be confirmed. In detail, interpolation may be performedby applying filtering (denoted int_func_1D as a filtering function) topixels (assuming that a filter is applied to an integer unit pixel inthis example) adjacent to an interpolation target pixel (denoted as x).Herein, since one reference pixel layer is used as the reference pixel,interpolation may be performed using adjacent pixels belonging to thesame reference pixel layer as the interpolation target pixel x.

Referring to FIG. 23b , when two or more reference pixel layers ref_i,ref_j, and ref_k are supported as reference pixels, a method ofobtaining interpolation pixels having a decimal unit may be confirmed.In FIG. 23b , when a reference pixel interpolation process is performedin the reference pixel layer ref_j, interpolation of the interpolationtarget pixel in a decimal unit may be performed by additionally usingother reference pixel layers ref_k and ref_i. Specifically, each of theinterpolation target pixel (location of xj) and pixels (xk, xi) atpositions (corresponding position in each reference pixel layeraccording to a direction of a prediction mode) belonging to otherreference pixel layers and corresponding to the interpolation targetpixel may be obtained by filtering (interpolation process. Int_func_1Dfunction) adjacent pixels (ak˜hk, aj˜hj, ai˜hi), and the finalinterpolation pixel x in the reference pixel layer ref_j may be obtainedby additional filtering (may be filtering that is not an interpolation,but is a weighted average such as [1, 2, 1]/4, [1, 6, 1]/8) on theobtained first interpolation pixels (xk, xj, xi). In this example, thepixels (xk, xi) of the other reference pixel layer corresponding to theinterpolation target pixel is assumed to be a decimal unit pixel thatmay be obtained through an interpolation process.

In the above example, though it was described as a case that the firstinterpolation pixels are obtained through filtering in each referencepixel layer and the final interpolation pixel is obtained by performingadditional filtering on the first interpolation pixels, adjacent pixels(ak˜hk, aj˜hj, ai˜hi) of various reference pixel layers may be filteredto obtain the final interpolated pixel at a time.

The layer used as an actual reference pixel among the three referencepixel layers supported in FIG. 23b may be the ref_j. That is, it may bethe case that another reference pixel layer included in a candidategroup (e.g., although not consisting of reference pixels may mean thatpixels of a reference pixel layer are not used for prediction, thepixels are referenced for interpolation in this case, so they may betechnically included in a case that the pixels are used for prediction)is used for interpolation of one reference pixel layer composed ofreference pixels.

Referring to FIG. 23c , an example of a case in which two supportedreference pixel layers are used as reference pixels is illustrated. Thefinal interpolation pixel x may be obtained by configuring pixels (di,dj, ei, ej in this example) adjacent to the decimal unit position to beinterpolated in each supported reference pixel layer as input pixels andperforming filtering on the adjacent pixels. However, as shown in FIG.23b , it may also be possible to obtain a first interpolation pixel ineach reference pixel layer and perform additional filtering on the firstinterpolation pixel to obtain the final interpolation pixel x.

The above example is not limited to the reference pixel interpolationprocess, and may be understood as a process combined with otherprocesses (e.g., reference pixel filter process, prediction blockgeneration process, etc.) of intra prediction.

FIGS. 24a and 24b are first exemplary diagrams for illustrating anadaptive reference pixel filtering method according to an embodiment ofthe present invention.

In general, though the reference pixel filter unit may perform smoothingusing a low pass filter {Low-pass Filter. e.g., a 3-tap, 5-tap filtersuch as [1, 2, 1]/4, [2, 3, 6, 3, 2]/16, etc.}, different types offilters (e.g., high pass filters, etc.) may be used depending on afilter application purpose {e.g. sharpening, etc.}. In the presentinvention, the filtering is performed for the purpose of smoothing toreduce deterioration occurring in an encoding/decoding process.

Reference pixel filtering may be determined according to anencoding/decoding setting. However, applying a presence or absence offiltering in a batch does not reflect partial characteristics of animage, and thus, filtering based on the partial characteristics of theimage may be advantageous for improving encoding performance. Herein,the characteristics of the image may be determined according to an imagetype, color component, quantization parameter, encoding/decodinginformation of a current block (e.g. size, shape, partition information,prediction mode, etc. of the current block) as well as encoding/decodinginformation of the neighboring block, a combination of encoding/decodinginformation of the current block and the neighboring block, and thelike. It may also be determined according to a reference pixeldistribution characteristic (e.g. dispersion, standard deviation, flatarea, discontinuous area, etc. of reference pixel area).

Referring to FIG. 24a , filtering may not be applied when belonging to aclassification (category 0) according to some encoding/decoding settings(e.g., block size range A, prediction mode B, color component C, etc.),and the filtering may be applied when belonging to a classification(category 1) according to some encoding/decoding settings (e.g.,prediction mode A of the current block, prediction mode B of a presetneighboring block, etc.).

Referring to FIG. 24b , filtering may not be applied when belonging to aclassification (category 0) according to some encoding/decoding settings(e.g., size A of the current block, size B of the neighboring block,prediction mode C of the current block, etc.), the filtering may beapplied when belonging to a classification (category 1) according tosome encoding/decoding settings (e.g., size A of the current block,shape B of the current block, size C of the neighboring block, etc.),and the filtering may be applied by using a filter A when belonging to aclassification (category 2) according to some encoding/decoding settings(e.g., parent block A of the current block, parent block B of theneighboring block, etc.).

Accordingly, whether to apply filtering, a type of filter, whether toencode filter information (explicit or implicit), the number offiltering, etc. may be determined according to a size of the currentblock and the neighboring block, a prediction mode, a color component,and the like. The type of filter may be classified into a difference inthe number of taps of a filter, a filter coefficient, and the like. Inthis case, when the number of times of filtering is applied to two ormore times, applications such as applying the same filter multiple timesor applying different filters may be possible.

The above example may be a case where reference pixel filtering is setin advance according to characteristics of an image. That is, it may bethe case that filter related information is implicitly determined.However, when the determination of the characteristics of the image asdescribed above is not correct, a coding efficiency may have a negativeeffect. Therefore, this part needs to be considered.

In order to prevent the above case, an explicit setting in referencepixel filtering may be possible. For example, information about whetherfiltering is applied may occur. In this case, filter selectioninformation does not occur when there is one filter, and the filterselection information may occur when multiple filter candidate groupsexists.

Although an implicit setting and an explicit setting have been describedwith reference to reference pixel filtering through the above example, amixed case in which an explicit setting is set in some cases and animplicit setting in some cases may be possible. The implicit meaninghere means that information (e.g., whether to apply filteringinformation and filter type information) related to a reference pixelfilter can be derived from a decoder.

FIG. 25 is a second exemplary diagram for illustrating an adaptivereference pixel filtering method according to an embodiment of thepresent invention.

Referring to FIG. 25, categories may be classified according tocharacteristics of an image identified through encoding/decodinginformation, and reference pixel filtering may be adaptively performedaccording to the classified categories.

For example, no filtering is applied when classified as category 0, andfiltering A is applied when classified as category 1. The categories 0and 1 may be examples of implicit reference pixel filtering.

In addition, when classified as category 2, filtering may not be appliedor filtering A may be applied. In this case, the generated informationmay be information on whether filtering is applied, and filter selectioninformation does not occur.

In addition, when classified as category 3, filtering A may be appliedor filtering B may be applied. In this case, the generated informationmay be filter selection information, and it may be an example wherefiltering is performed unconditionally. That is, when classified ascategory 3, it may be understood as a situation in which filtering mustbe performed but a type of filtering must be selected.

In addition, when classified as category 4, filtering may not beapplied, filtering A may be applied, or filtering B may be applied. Inthis case, the generated information may be information on whetherfiltering is applied and filter selection information.

In summary, an explicit or implicit process may be determined accordingto a category, and a candidate group setting related to each referencepixel filter may be adaptively configured when the explicit process isperformed.

The following example may be considered with respect to the category.

First, in the case of a block having a size of 64×64 or more, one of<Filtering off>, <filtering on+filter A>, <filtering on+filter B>, and<filtering on+filter C> may be implicitly determined according to aprediction mode of a current block. In this case, when consideringreference pixel distribution characteristics, an additional candidatemay be <filtering on+filter C>. That is, when filtering is on, filter A,filter B, or filter C may be applied.

In addition, for a block having a size of less than 64×64 and 16×16 ormore, one of <filtering off>, <filtering on+filter A>, and <filteringon+filter B> may be implicitly determined according to the predictionmode of the current block.

In addition, in the case of a block having a size of less than 16×16,one of <filtering off>, <filtering on+filter A>, and <filteringon+filter B> is selected according to the prediction mode of the currentblock. In some prediction modes, it is implicitly determined as<filtering off>, and in some prediction modes, one of <filtering off>and <filtering on+filter A> is explicitly selected, and in someprediction modes, <filtering off> and <filtering+filter B> may beexplicitly selected.

As an example of explicit processing of multiple reference pixel filterrelated information, when reference pixels acquired according to eachfiltering (In this example, the filtering off situation is alsoincluded) are the same or similar, generating the reference pixel filterinformation (e.g., reference pixel filtering allowance information,reference pixel filter information, etc.) may be a result of generatingunnecessary redundant information. For example, reference pixel filterrelated information may be omitted if reference pixel distributioncharacteristics (e.g., characteristics determined by comparing athreshold value with a value obtained through an average, variance, etc.of each reference pixel) obtained by each filtering are the same orsimilar. If the reference pixel filter related information is omitted,filtering may be applied by a preset method (e.g., filtering off). Inthe decoder, the same determination as that of the encoder may beperformed as to whether to receive the reference pixel filter relatedinformation by receiving intra prediction information, and based on thisdetermination, it may be determined whether to receive the referencepixel filter related information.

Assuming information is explicitly generated in relation to referencepixel filtering, indication information(adaptive_ref_filter_enabled_flag in this example) that allows adaptivereference pixel filtering may occur on the basis of a video, a sequence,a picture, a slice, a tile, and the like.

If the indication information means to allow adaptive reference pixelfiltering (adaptive_ref_filter_enabled_flag=1 in this example),allowance information of the adaptive reference pixel filtering(adaptive_ref_filter_flag in this example) may occur on the basis of apicture, a slice, a tile, a block, and the like.

If the allowance information means adaptive reference pixel filtering(adaptive_ref_filter_flag=1 in this example), the reference pixelfiltering related information may be generated on the basis of apicture, a slice, a tile, and a block.

In this case, when the adaptive reference pixel filtering is not allowedor the adaptive reference pixel filtering is not applied, the filteringoperation of the reference pixel may be determined according to apredetermined setting (as described above, whether to apply filtering,the type of filter, etc. are determined according to imageencoding/decoding information in advance).

FIGS. 26a to 26b are exemplary diagrams of using one reference pixellayer in reference pixel filtering according to an embodiment of thepresent invention.

Referring to FIG. 26a , it may be confirmed that interpolation isperformed by applying filtering (called smt_func_1 function) to a targetpixel d and pixels (a, b, c, e, f, g) adjacent to the target pixel damong pixels belonging to the reference pixel layer ref_i.

Although FIG. 26a may be an example in which one filtering is generallyapplied, the case that a plurality of filtering is applied is also bepossible. For example, the second filtering may be applied to areference pixel obtained by applying the first filtering.

Referring to FIG. 26b , a filtered (called smt_func_2 function) pixele*may be obtained through linear interpolation proportional to distance(e.g., distance z to a) with respect to pixels located at both sides ofthe target pixel e. Herein, the pixels located at both sides may bepixels located at both end points of adjacent pixels in a block whichconsists of the top block, the left block, the top block+the top rightblock, the left block+the bottom left block, the top left block+the topblock+the top right block, the top left block+the left block+the bottomleft block, the top left block+the left block+the top block+the bottomleft block+the top right block, etc. of the current block. FIG. 26b maybe reference pixel filtering performed according to reference pixeldistribution characteristics.

FIGS. 26a to 26b illustrate a case in which pixels of the same referencepixel layer as a filtering target reference pixel are used for referencepixel filtering. In this case, the type of filter used for the referencepixel filtering may or may not be the same according to the referencepixel layer.

In addition, when using multiple reference pixel layers, it may bepossible to use not only pixels of the same reference pixel layer butalso pixels of other reference pixel layers for reference pixelfiltering in some reference pixel layers.

FIG. 27 is an exemplary diagram of using multiple reference pixel layersin reference pixel filtering according to an embodiment of the presentinvention.

Referring to FIG. 27, first, filtering may be performed using pixelsbelonging to the same reference pixel layer in the reference pixellayers ref_k and ref_i, respectively. That is, the filtered pixel dk*may be obtained by performing filtering (defined by functionsmt_func_1D) on the target pixel dk and its adjacent pixels ak to gk inthe reference pixel layer ref_k, and the filtered pixel di* may beobtained by performing filtering (defined by function smt_func_1D) onthe target pixel di and its adjacent pixels ai to gi in the referencepixel layer ref_i.

In addition, when performing reference pixel filtering in the referencepixel layer ref_j, not only the same reference pixel layer ref_j butalso pixels belonging to other reference pixel layers ref_i and ref_kspatially adjacent to the reference pixel layer ref_j may be used. Indetail, the interpolated pixel dj* may be obtained by applying filtering(defined by smt_func_2D function) on spatially adjacent pixels (ck, dk,ek, cj, ej, ci, di, ei. that may be a filter having a 3×3 square mask)around the target pixel dj. However, the present invention is notlimited to a 3×3 square shape, and a filter having a mask such as a 5×2rectangular shape (bk, ck, dk, ek, fk, bj, cj, ej, fj), a 3×3 diamondshape (dk, cj, ej, di), or a 5×3 cross shape (dk, bj, cj, ej, fj, di)may be used around target pixel.

Herein, a reference pixel layer is composed of pixels belonging to aneighboring block adjacent to the current block and close to a boundaryof a current block, as shown in FIGS. 18 to 22 b. In consideration ofthis aspect, filtering using pixels in the same reference pixel layer inthe reference pixel layer ref_k and the reference pixel layer ref_i mayapply a filter in the form of a one-dimensional mask using pixels thatare horizontally or vertically adjacent to an interpolation targetpixel. However, an interpolation pixel for the reference pixel dj in thereference pixel layer ref_j may be obtained by applying a filter in theform of a two-dimensional mask using all adjacent pixels in spatiallyup/down/left/right directions.

In addition, the second reference pixel filtering may be applied to areference pixel to which the first reference pixel filtering is appliedin each reference pixel layer. For example, the first reference pixelfiltering may be performed using reference pixels included in eachreference pixel layer ref_k, ref_j, and ref_i, and in the referencepixel layer (referred to as ref_k*, ref_j*, and ref_i*) where the firstreference pixel filtering is performed, reference pixel filtering may beperformed using reference pixels of not only respective reference pixellayers but also other reference pixel layers.

The prediction block generation unit may generate a prediction blockaccording to at least one intra prediction mode (or may be referred tosimply as a prediction mode), and may use a reference pixel based on theprediction mode. In this case, the prediction block may be generated byextrapolating, interpolating, averaging (DC mode), or copying thereference pixel according to the prediction mode. Herein, theextrapolation may be applied to a directional mode among intraprediction modes, and the rest may be applied to a non-directional mode.

Meanwhile, when copying reference pixels, one or more prediction pixelsmay be generated by copying one reference pixel to multiple pixels in aprediction block, one or more prediction pixels may be generated bycopying one or more reference pixels, and the number of the copiedreference pixels may be equal to or less than the number of the copiedprediction pixels. One or more prediction pixels may be generated bycopying one or more reference pixels, and the number of copied referencepixels may be equal to or less than the number of copied predictionpixels.

In addition, although one prediction block may be generally generatedfor each prediction according to one intra prediction mode, the finalprediction block may be generated by obtaining multiple predictionblocks and applying a weighted sum to the obtained plurality ofprediction blocks. Herein, the plurality of prediction blocks may meanprediction blocks obtained according to a reference pixel layer.

The prediction mode determination unit of an encoding apparatus performsa process for selecting an optimal mode among multiple prediction modecandidate groups. In general, an optimal mode may be determined in termsof encoding cost by using a rate-distortion technique that predicts anamount of bits generated according to a block distortion {e.g.,Distortion of the current block and the restore block. Sum of AbsoluteDifference (SAD), Sum of Square Difference, etc.} and a prediction mode.The prediction block generated based on the prediction mode determinedthrough the above process may be transmitted to the subtractor and theadder (at this time, since a decoding apparatus may obtain informationindicating the optimal prediction mode from the encoding apparatus, theprocess of selecting the optimal prediction mode may be omitted).

The prediction mode encoding unit of an encoding apparatus may encode anoptimal intra prediction mode selected through the prediction modedetermination unit. In this case, index information indicating anoptimal prediction mode may be encoded as it is, or a prediction modemay be predicted through a prediction mode that can be obtained fromanother block, and prediction information (e.g., difference between apredicted prediction mode index and a prediction mode index of thecurrent block) about the prediction mode may be encoded. Herein, theformer case may be applied to a chrominance component and the lattercase to a luminance component.

In the case of predicting and encoding an optimal prediction mode for acurrent block, a prediction value (or prediction information) for theprediction mode may be referred to as Most Probable Mode (MPM). In thiscase, the MPM is a prediction mode that is likely to be the optimalprediction mode for the current block, and may be configured as a presetprediction mode (e.g. DC, Planar, Vertical, Horizontal, Diagonal mode,etc.) or a prediction mode of a spatially adjacent block (e.g., left,top, top left, top right, bottom left block, etc.). Herein, the diagonalmode may mean a Diagonal up right, a Diagonal down right, and a Diagonaldown left, and may correspond to modes 2, 18, and 34 of FIG. 17.

In addition, a mode derived from a prediction mode included in an MPMcandidate group, which is a set of prediction modes that may beconfigured as the MPM, may be added as the MPM candidate group. In thecase of a directional mode, the MPM candidate group may include aprediction mode having a preset index interval from the prediction modeincluded in the MPM candidate group. For example, in FIG. 17, when mode10 is a mode included in the MPM candidate group, it may correspond to amode in which modes 9, 11, 8, 12, etc. are derived.

The above example may correspond to a case in which the MPM candidategroup is configured in multiple modes, and the MPM candidate groupconfiguration (e.g., number of prediction modes included in the MPMcandidate group, configuration priority) is determined according to anencoding/decoding setting (e.g., prediction mode candidate group, imagetype, block size, block shape, etc.) and may include at least one mode.

The priority of prediction modes to be included in an MPM candidategroup may be set. The order of prediction modes to be included in theMPM candidate group may be determined according to set priority, and theMPM candidate group configuration may be completed by filling theprediction modes by a predetermined number. Herein, the priority may beset in order of a prediction mode of a block spatially adjacent to acurrent block to be predicted, a preset prediction mode, and a modederived from a prediction mode first included in the MPM candidategroup, but is not limited thereto.

More specifically, a priority may be set in the order of theleft—top—bottom left—top right—top left blocks among spatially adjacentblocks, the priority may be set in the order ofDC—Planar—Vertical—Horizontal mode among preset prediction modes, and anMPM candidate group may include a prediction mode obtained by adding +1,−1, and the like (an integer value) to an index value according to theprediction mode included in the MPM candidate group (corresponds to aprediction mode number according to FIG. 17). As one of such examples,the priority may be set in the order of left—top—DC—Planar—bottomleft—top right—top left—(spatial adjacent block mode+1)−(spatialadjacent block mode−1)˜horizontal—vertical—diagonal modes.

Although the priority of the configuration of the MPM candidate group isfixed in the above example, the priority may be adaptively determinedaccording to a shape, size, and the like of the block.

When a prediction mode encoding of a current block is performed usingMPM, information (e.g., most_probable_mode_flag) about whether theprediction mode matches the MPM may be generated.

When matching with the MPM (e.g., most_probable_mode_flag=1), MPM indexinformation (e.g., mpm_idx) may be additionally generated according to aconfiguration of the MPM. For example, if the MPM is configured with oneprediction mode, no additional MPM index information may be generated.If the MPM is configured with multiple prediction modes, indexinformation corresponding to the prediction mode of the current blockmay be generated in the MPM candidate group.

When not matching with the MPM (e.g., most_probable_mode_flag=0),non-MPM index information (e.g., non_mpm_idx) corresponding to aprediction mode of the current block may be generated in a predictionmode candidate group (referred to as a non-MPM candidate group) otherthan the MPM candidate group among supported intra prediction modes.This may be an example of the case where the non-MPM is composed of onegroup.

When a non-MPM candidate group is composed of multiple groups,information about which group a prediction mode of a current blockbelongs to may be generated. For example, when the non-MPM is composedof groups A and B, if the prediction mode of the current block matches aprediction mode of the group A (e.g, non_mpm_A_flag=1), indexinformation corresponding to the prediction mode of the current blockmay be generated in a candidate group of the A group. If there is nomatch (e.g., non_mpm_A_flag=0), index information corresponding to theprediction mode of the current block may be generated in a remainingprediction mode candidate group (or a candidate group of B group). As inthe above example, the non-MPM may be composed of multiple groups, andthe number of groups may be determined according to a prediction modecandidate group. For example, when a prediction mode candidate group is35 or less, it may be one, or in other cases, two or more.

In this case, a specific A group may be composed of modes that aredetermined to have a high probability of matching a prediction mode of acurrent block except for an MPM candidate group. For example, predictionmodes of the next rank that are not included in the MPM candidate groupmay be included in the A group, or directional modes having regularintervals may be included in the A group.

When a non-MPM is composed of multiple groups as in the above example,when the number of prediction modes is large and the prediction mode ofthe current block does not match an MPM, the amount of mode coding bitsmay be reduced.

When performing prediction mode encoding (or prediction mode decoding)of a current block using an MPM, a binarization table applied to eachprediction mode candidate group (e.g., MPM candidate group, non-MPMcandidate group, etc.) may be generated separately, and a binarizationmethod applied to each candidate group may also be applied individually.

In the above examples, terms such as the MPM candidate group, thenon-MPM candidate group, and the like are not limited thereto but onlysome terms used in the present invention. In detail, it is representedby information about which category belongs to and mode information inthe category by classifying a current intra prediction mode intomultiple categories. As another expression, it may also be possible touse terms such as the first MPM candidate group, the second MPMcandidate group, and the like.

FIG. 28 is a block diagram for illustrating a encoding/decoding methodfor an intra prediction mode according to an embodiment of the presentinvention.

Referring to FIG. 28, first, mpm_flag is acquired (S10) to confirmwhether it matches the first MPM (indicated by the mpm_flag) (S11), andif it matches, MPM index information (mpm_idx) is checked (S12). If itdoes not match the first MPM, rem_mpm_flag is acquired (S13) to confirmwhether it matches the second MPM (indicated by the rem_mpm_flag), andif it matches, the second MPM index information (rem_mpm_idx) is checked(S16). If the second MPM does not match, index information(rem_mode_idx) for a candidate group configured with remainingprediction modes may be checked (S15). In the present example, althoughthe index information generated according to whether the second MPM ismatched is represented by the same syntax element, another mode encodingsetting (for example, a binarization method or the like) may be applied,or the index information may be set differently and processed.

In the image decoding method according to an embodiment of the presentinvention, an intra prediction may be configured as follows. The intraprediction of a prediction unit may include decoding a prediction mode,constructing a reference pixel, and generating a prediction block. Inaddition, the image decoding apparatus may be configured to include aprediction mode decoding unit for decoding the prediction mode, areference pixel construction unit for constructing the reference pixel,and a prediction block generation unit for generating the predictionblock. Some of the processes described above may be omitted or otherprocesses may be added, and may be changed in an order other than theorder described above

Since the reference pixel construction unit and the prediction blockgeneration unit of the image decoding apparatus play the same role asthe corresponding components of the image encoding apparatus, a detaileddescription thereof will be omitted. In addition, the prediction modedecoding unit may be performed by using the inverse method used by theprediction mode encoding unit.

Hereinafter, various embodiments of intra prediction according to areference pixel configuration of a decoding apparatus will be describedwith reference to FIGS. 29 to 31. In this case, a description about thesupport of the reference pixel layer and the reference pixel filteringmethod described above with reference to the drawings should beinterpreted to be equally applicable to a decoding apparatus, anddetailed descriptions thereof will be omitted in order to preventduplication.

FIG. 29 is a first exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

In the first exemplary diagram according to FIG. 29, It is assumed thatmultiple reference pixel layers are supported, at least one referencepixel layer among the supported reference pixel layers is used asreference pixels, and multiple candidate groups for reference pixelfiltering are supported and one of them is selected.

After an encoder configures multiple reference pixel layers as referencepixels candidate group (in this example, completed until the process ofreference pixel generation), reference pixel filtering and referencepixel interpolation are applied after configuring at least one referencepixel layer as reference pixels. At this time, multiple candidate groupsfor reference pixel filtering are supported.

A process for selecting an optimal mode among prediction mode candidategroups is performed. When the optimal prediction mode is determined, aprediction block according to the corresponding mode is generated andtransmitted to the subtractor, and an encoding process for intraprediction information is performed. In this example, it is assumed thata reference pixel layer and reference pixel filtering are implicitlydetermined according to encoding information.

The decoder restores intra prediction information (e.g., predictionmode, etc.), generates a prediction block according to the reconstructedprediction mode, and delivers the prediction block to the subtractor. Inthis case, it is assumed that a reference pixel layer and referencepixel filtering for generating the prediction block are implicitlydetermined.

Referring to FIG. 29, a bitstream may be configured in one intraprediction mode (intra mode) (S20). In this case, the reference pixellayer ref_idx and the reference pixel filtering category ref_filter_idxthat are supported (or used) in the current block may be implicitlydetermined (determined as Categories A and B, respectively, S21 to S22)according to the intra prediction mode. However, at this time,encoding/decoding information (e.g., image type, color component, blocksize, shape, etc.) may be additionally considered.

FIG. 30 is a second exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

In the second exemplary diagram according to FIG. 30, it is assumed thatmultiple reference pixel layers are supported, and one reference pixellayer is used as reference pixels among multiple supported referencepixel layers. In addition, it is assumed that multiple candidate groupsfor reference pixel filtering are supported and one of the filters isselected. The difference from FIG. 29 is that the encoding apparatusexplicitly generates the information about the selection.

If an encoder determines that multiple reference pixel layers aresupported, reference pixel filtering and reference pixel interpolationare applied after one reference pixel layer is configured as referencepixels. In this case, multiple filtering methods related to referencepixel filtering are supported.

When the process of determining an optimal prediction mode for a currentblock in an encoder is performed, selecting an optimal reference pixellayer and optimal reference pixel filtering may be further consideredfor each prediction mode. When the optimal prediction mode, the optimalreference pixel layer, and reference pixel filtering for the currentblock are determined, the generated prediction block is transferred tothe subtractor and an encoding process for intra prediction informationis performed.

The decoder restores intra prediction information (e.g., information ofprediction mode, reference pixel layer, reference pixel filtering,etc.), generates a prediction block using the reconstructed information,and delivers the prediction block to the subtractor. In this case, thereference pixel layer and the reference pixel filtering for generatingthe prediction block depend on a setting determined according to theinformation transmitted from an encoder.

Referring to FIG. 30, a decoder checks an optimal prediction mode for acurrent block through intra prediction mode (intra mode) informationincluded in a bitstream (S30), and determine whether multiple referencepixel layers are supported (multi ref flag) (S31). If multiple referencepixel layers are supported, a reference pixel layer used for intraprediction may be determined by checking the reference pixel layerselection information ref_idx (S32). If multiple reference pixel layersare not supported, the process of obtaining reference pixel layerselection information ref_idx (S32) may be omitted.

Next, it is determined whether adaptive reference pixel filtering issupported (adap_ref_smooth_flag) (S33), and if the adaptive referencepixel filtering is supported, a filtering method for a reference pixelis determined through reference pixel filter information(ref_filter_idx) (S34).

FIG. 31 is a third exemplary diagram for illustrating a bitstreamconfiguration for intra prediction according to a reference pixelconfiguration.

In the third exemplary view according to FIG. 31, it is assumed thatmultiple reference pixel layers are supported and one reference pixellayer is used among multiple reference pixel layers. It is also assumedthat multiple candidate groups for reference pixel filtering aresupported and one of them is selected. The difference from FIG. 30 isthat the selection information is adaptively generated.

Reference pixel filtering and reference pixel interpolation are appliedafter configuring reference pixels using one reference pixel layer amongmultiple reference pixel layers supported by an encoder. In this case, aplurality of filtering related to the reference pixel filtering issupported.

When a process for selecting an optimal mode among multiple predictionmode candidate groups are performed, selecting an optimal referencepixel layer and selecting an optimal reference pixel filtering may befurther considered for each prediction mode. When an optimal predictionmode, reference pixel layer, and reference pixel filtering aredetermined, a prediction block is generated and transmitted to thesubtractor, and an encoding process of intra prediction relatedinformation is performed.

In this case, if a redundancy of the generated prediction block ischecked, and if it is the same as or similar to a prediction blockobtained by using another reference pixel layer, selection informationon the optimal reference pixel layer may be omitted and a presetreference pixel layer may be used. In this case, the preset referencepixel layer may be a layer closest to the current block.

For example, the redundancy may be determined based on a differencevalue (distortion value) between the prediction block generated throughref_0 and the prediction block generated through ref_1 of FIG. 19C. Ifthe difference value is smaller than a preset threshold, it may bedetermined that there is redundancy of the prediction block, otherwiseit may be determined that there is no redundancy of the predictionblock. In this case, the threshold may be adaptively determinedaccording to a quantization parameter.

In addition, if optimal reference pixel filtering information alsochecks a redundancy of a prediction block and is the same or similar toa prediction block obtained by applying another reference pixelfiltering, the reference pixel filtering information may be omitted andpreset reference pixel filtering may be applied.

For example, it is determined whether a redundancy is based on adifference value between the prediction block obtained through filteringA (3-tap filter in this example) and the prediction block obtainedthrough filtering B (5-tap filter in this example). In this case, whenthe difference value is smaller than a preset threshold value, it may bedetermined that there is a redundancy of a prediction block. if there isthe redundancy of the prediction block, the prediction block may begenerated through a preset reference pixel filtering method. Herein, thepreset reference pixel filtering may be a filtering method having a lowtap number or a low complexity, and may include a case in whichfiltering application is omitted.

The decoder reconstructs intra prediction related information (e.g.,prediction mode, reference pixel layer information, reference pixelfiltering information, etc.), generates the corresponding predictionblock and transmits it to the subtractor. In this case, reference pixellayer information and reference pixel filtering for generating aprediction block are based on a setting determined according toinformation transmitted from an encoder, and the decoder directly checks(not through syntax elements) whether there is a redundancy, and ifthere is the redundancy, a preset method is used.

Referring to FIG. 31, a decoder may first check intra prediction mode(intra mode) information about a current block (S40) and check whethermultiple reference pixel layers (multi-ref flag) are supported (S41). Ifmultiple reference pixel layers are supported, a redundancy of theprediction block according to multiple supported reference pixel layersare checked (indicated by a ref check process, S42). If there is noredundancy of the prediction block as a result of the redundancy check(redund_ref=0, S43), the optimal reference pixel layer may be determinedby referring to the selection information ref_idx for the referencepixel layer from the bit stream (S44).

Next, whether adaptive reference pixel filtering is supported(adap_ref_smooth_flag) is checked (S45), and if the adaptive referencepixel filtering is supported, a redundancy of a prediction blockaccording to multiple supported reference pixel filtering methods arechecked (ref check process, S46). If there is no redundancy of theprediction block (redund_ref=0, S47), an optimal reference pixelfiltering method may be determined by referring to selection information(ref_filter_idx) of a reference pixel filtering method from a bitstream(S48).

In this case, in the drawing, redund_ref is a value indicating a resultof redundancy check and a value of 0 indicates that there is noredundancy.

In addition, when there is a redundancy of a prediction block, a decodermay perform intra prediction using a preset reference pixel layer and apreset reference pixel filtering method.

FIG. 32 is a flowchart for illustrating an image decoding methodsupporting multiple reference pixel layers according to an embodiment ofthe present invention.

Referring to FIG. 32, an image decoding method supporting multiplereference pixel layers may include checking whether a bitstream supportsmultiple reference pixel layers (S100), determining a reference pixellayer to be used for a current block by referring to syntax informationincluded in the bitstream if the plurality of reference pixel layers aresupported (S110), constructing a reference pixel using a pixel belongingto the determined reference pixel layer (S120), and performing intraprediction on the current block by using the constructed reference pixel(S130).

Herein, after the checking of whether the plurality of reference pixellayers are supported (S100), the method may further include checkingwhether the bitstream supports an adaptive reference pixel filteringmethod.

Herein, after checking whether the plurality of reference pixel layersis supported (S100), if the plurality of reference pixel layers is notsupported, the method may include configuring a reference pixel usingthe preset reference pixel layer.

The instructions that can be executed by various computer means andrecorded on a computer readable medium. Computer-readable media mayinclude, alone or in combination with program instructions, data files,data structures, and the like. The program instructions recorded on thecomputer readable medium may be those specially designed and constructedfor the present invention, or may be known and available to thoseskilled in computer software.

Examples of computer readable media may include hardware devicesspecifically configured to store and execute program instructions, suchas ROM, RAM, flash memory, and the like. Examples of programinstructions may include high-level language code that can be executedby a computer using an interpreter as well as machine code such asproduced by a compiler. The hardware device described above may beconfigured to operate with at least one software module to perform theoperations of the present invention, and vice versa.

In addition, the above-described method or apparatus may be implementedby combining all or part of the configuration or function, or may beimplemented separately.

Although described above with reference to a preferred embodiment of thepresent invention, those skilled in the art will appreciate that variousmodifications and changes can be made in the present invention withoutdeparting from the spirit and scope of the invention as set forth in theclaims below.

The invention claimed is:
 1. A method for decoding an image using adivision unit, comprises: dividing an encoded image included in areceived bitstream into at least one division unit by referring to asyntax obtained from the received bitstream; deriving an intraprediction mode of a current block in the division unit; deriving areference sample layer of the current block from a plurality ofreference sample layers; generating reference samples based on thederived reference sample layer; and generating a prediction sample ofthe current block based on the reference samples and the intraprediction mode, wherein the plurality of reference sample layers areincluded in reconstructed neighboring blocks of the current block, andwherein each of the reference sample layers of the current block has adifferent distance from a boundary of the current block.
 2. The methodof claim 1, wherein when the intra prediction mode is a non-directionalmode, the prediction sample is generated by using only the referencesample layer having a shortest distance from the boundary of the currentblock.
 3. The method of claim 2, wherein the non-directional mode isrepresentative of a planar mode.
 4. The method of claim 1, wherein thereference sample layer is derived based on a color component.
 5. Themethod of claim 1, wherein a number of the plurality of reference samplelayers is equal to or more than
 3. 6. A method for encoding an imageusing a division unit, comprises: dividing the image into at least onedivision unit; determining an intra prediction mode of a current blockin the division unit; determining a reference sample layer of thecurrent block from a plurality of reference sample layers; generatingreference samples based on the determined reference sample layer; andgenerating a prediction sample of the current block based on thereference samples and the intra prediction mode, wherein the pluralityof reference sample layers are included in reconstructed neighboringblocks of the current block, and wherein each of the reference samplelayers of the current block has a different distance from a boundary ofthe current block.
 7. The method of claim 6, wherein when the intraprediction mode is a non-directional mode, the prediction sample isgenerated by using only the reference sample layer having a shortestdistance from the boundary of the current block.
 8. The method of claim7, wherein the non-directional mode is representative of a planar mode.9. The method of claim 6, wherein the reference sample layer isdetermined based on a color component.
 10. The method of claim 6,wherein a number of the plurality of reference sample layers is equal toor more than
 3. 11. A non-transitory computer readable storage mediumstoring information causing a decoding apparatus to perform operationscomprising: dividing an encoded image included in a received bitstreaminto at least one division unit by referring to a syntax obtained fromthe received bitstream; deriving an intra prediction mode of a currentblock in the division unit; deriving a reference sample layer of thecurrent block from a plurality of reference sample layers; generatingreference samples based on the derived reference sample layer; andgenerating a prediction sample of the current block based on thereference samples and the intra prediction mode, wherein the pluralityof reference sample layers are included in reconstructed neighboringblocks of the current block, and wherein each of the reference samplelayers of the current block has a different distance from a boundary ofthe current block.